diff --git a/php/class-delivery.php b/php/class-delivery.php
index a4eaa7452..f2ec5d184 100644
--- a/php/class-delivery.php
+++ b/php/class-delivery.php
@@ -1583,9 +1583,10 @@ public function parse_element( $element, $content = '' ) {
 		// Track back the found URL.
 		if ( $this->media->is_cloudinary_url( $raw_url ) ) {
 			$public_id = $this->media->get_public_id_from_url( $raw_url );
-			if ( isset( $this->known[ $public_id ] ) ) {
-
-				$url               = $this->known[ $public_id ]['sized_url'];
+			// A URL with transformations of its own (e.g. typed into content by hand) is left as is, since rebuilding it would drop them.
+			if ( isset( $this->known[ $public_id ] ) && ! $this->has_custom_transformations( $raw_url, $this->known[ $public_id ]['post_id'] ) ) {
+				// The relation's sized_url is relative to the content dir, while the URL keys of $this->known are full URLs.
+				$url               = $this->get_searchable_url( $this->known[ $public_id ] );
 				$attributes['src'] = $url;
 			}
 		}
@@ -1837,25 +1838,54 @@ public function validate_url( $url ) {
 	}
 
 	/**
-	 * Set url usability.
+	 * Check if a Cloudinary URL has transformations other than the ones the plugin applies to the attachment itself:
+	 * its size, and the attachment's own and global transformations.
 	 *
-	 * @param array     $item      The item result.
-	 * @param null|bool $auto_sync If auto_sync is on.
+	 * @param string $url           The Cloudinary URL.
+	 * @param int    $attachment_id The attachment ID.
+	 *
+	 * @return bool
 	 */
-	protected function set_usability( $item, $auto_sync = null ) {
+	protected function has_custom_transformations( $url, $attachment_id ) {
+		$type     = $this->media->get_resource_type( $attachment_id );
+		$expected = $this->media->get_transformations_from_string( (string) $this->media->cloudinary_url( $attachment_id ), $type );
+
+		foreach ( $this->media->get_transformations_from_string( $url, $type ) as $transformation ) {
+			// Loose comparison, so the order of the parameters doesn't matter.
+			if ( ! $this->is_size_transformation( $transformation ) && ! in_array( $transformation, $expected ) ) { // phpcs:ignore WordPress.PHP.StrictInArray.MissingTrueStrict
+				return true;
+			}
+		}
 
-		/**
-		 * Filter the found item to allow usability to be altered.
-		 *
-		 * @hook   cloudinary_set_usable_asset
-		 * @since  3.0.2
-		 *
-		 * @param array $item The found asset.
-		 *
-		 * @return array
-		 */
-		$item = apply_filters( 'cloudinary_set_usable_asset', $item );
+		return false;
+	}
 
+	/**
+	 * Check if a transformation is one the plugin uses to size an image, e.g. `w_300,h_200,c_scale` or `w_150,h_150,c_thumb,g_auto`.
+	 *
+	 * @param array $transformation The transformation, as parsed by Media::get_transformations_from_string().
+	 *
+	 * @return bool
+	 */
+	protected function is_size_transformation( $transformation ) {
+		if ( array_diff_key( $transformation, array_flip( array( 'width', 'height', 'crop', 'gravity' ) ) ) ) {
+			return false;
+		}
+		if ( isset( $transformation['crop'] ) && ! in_array( $transformation['crop'], array( 'scale', 'fill', 'thumb' ), true ) ) {
+			return false;
+		}
+
+		return ! isset( $transformation['gravity'] ) || 'auto' === $transformation['gravity'];
+	}
+
+	/**
+	 * Get the URL an item is known by, as used for the keys of $this->known.
+	 *
+	 * @param array $item The item result.
+	 *
+	 * @return string
+	 */
+	protected function get_searchable_url( $item ) {
 		/**
 		 * The URL to be searched for and prepared to be delivered by Cloudinary.
 		 *
@@ -1887,7 +1917,30 @@ protected function set_usability( $item, $auto_sync = null ) {
 		 *
 		 * @return string
 		 */
-		$url = apply_filters( 'cloudinary_delivery_searchable_url', $url, $item, $content_url );
+		return apply_filters( 'cloudinary_delivery_searchable_url', $url, $item, $content_url );
+	}
+
+	/**
+	 * Set url usability.
+	 *
+	 * @param array     $item      The item result.
+	 * @param null|bool $auto_sync If auto_sync is on.
+	 */
+	protected function set_usability( $item, $auto_sync = null ) {
+
+		/**
+		 * Filter the found item to allow usability to be altered.
+		 *
+		 * @hook   cloudinary_set_usable_asset
+		 * @since  3.0.2
+		 *
+		 * @param array $item The found asset.
+		 *
+		 * @return array
+		 */
+		$item = apply_filters( 'cloudinary_set_usable_asset', $item );
+
+		$url = $this->get_searchable_url( $item );
 
 		$found = array();
 
diff --git a/tests/phpunit/tests/test-delivery-parse-element.php b/tests/phpunit/tests/test-delivery-parse-element.php
new file mode 100644
index 000000000..492f6e2a1
--- /dev/null
+++ b/tests/phpunit/tests/test-delivery-parse-element.php
@@ -0,0 +1,220 @@
+<?php
+/**
+ * Tests for Cloudinary\Delivery::parse_element().
+ *
+ * Since WPP-1183, image_downsize() and wp_get_attachment_image_src() return Cloudinary URLs on
+ * the front end too, so tags built by theme code (wp_get_attachment_image() in a template, a
+ * shortcode, a widget) reach Delivery with a Cloudinary src instead of a local one. Delivery
+ * tracks such a src back to its attachment through the public ID; this covers that the track-back
+ * resolves to the same full URL the known items are keyed by, rather than the relation's
+ * content-relative sized_url, which never matched and left the tag unprocessed (no lazy loading,
+ * responsive breakpoints or SEO attributes). A src with transformations of its own (e.g. typed
+ * into content by hand) is left alone, as rebuilding it from the attachment would drop them.
+ *
+ * The known items are seeded directly rather than driven through prepare_delivery() and the
+ * relationship table, so no Cloudinary account is needed.
+ *
+ * @package Cloudinary
+ */
+
+/**
+ * Covers Delivery::parse_element().
+ */
+class Test_Delivery_Parse_Element extends WP_UnitTestCase {
+
+	/**
+	 * The Cloudinary base URL used for the test media component.
+	 *
+	 * @var string
+	 */
+	const BASE_URL = 'https://res.cloudinary.com/test-cloud';
+
+	/**
+	 * A real attachment, so the tracked-back item points at an existing post.
+	 *
+	 * @var int
+	 */
+	protected static $attachment_id;
+
+	/**
+	 * Create a real attachment once for all tests.
+	 *
+	 * @param WP_UnitTest_Factory $factory The test factory.
+	 *
+	 * @return void
+	 */
+	public static function wpSetUpBeforeClass( WP_UnitTest_Factory $factory ) {
+		self::$attachment_id = $factory->attachment->create_upload_object( DIR_TESTDATA . '/images/canola.jpg' );
+	}
+
+	/**
+	 * Build a Delivery instance using a media component pointed at the test Cloudinary domain.
+	 *
+	 * @return Test_Delivery_Parse_Element_Delivery
+	 */
+	protected function get_delivery() {
+		$plugin          = \Cloudinary\get_plugin_instance();
+		$media                      = new Test_Delivery_Parse_Element_Media( $plugin );
+		$media->base_url            = self::BASE_URL;
+		$media->filter              = new \Cloudinary\Media\Filter( $media );
+		$media->stub_cloudinary_url = self::BASE_URL . '/images/f_auto,q_auto/v1/canola/canola.jpg';
+
+		$delivery = new Test_Delivery_Parse_Element_Delivery( $plugin );
+		$delivery->set_media( $media );
+
+		return $delivery;
+	}
+
+	/**
+	 * Seed the known items the same way Delivery::set_usability() keys them: by public ID, and
+	 * by the full URL built from the content URL and the relation's content-relative sized_url.
+	 *
+	 * @param Test_Delivery_Parse_Element_Delivery $delivery The delivery instance.
+	 *
+	 * @return string The full URL the item is known by.
+	 */
+	protected function seed_known( $delivery ) {
+		$file = get_post_meta( self::$attachment_id, '_wp_attached_file', true );
+		$item = array(
+			'post_id'    => self::$attachment_id,
+			'public_id'  => 'canola',
+			'sized_url'  => '/uploads/' . $file,
+			'sync_type'  => 'media',
+			'format'     => 'jpg',
+			'width'      => 640,
+			'height'     => 480,
+			'post_state' => 'enable',
+		);
+		$url  = \Cloudinary\Utils::clean_url( content_url() ) . $item['sized_url'];
+
+		$delivery->known['canola'] = $item;
+		$delivery->known[ $url ]   = $item;
+
+		return $url;
+	}
+
+	/**
+	 * An img tag with a Cloudinary src for a known public ID is tracked back to its attachment.
+	 *
+	 * @return void
+	 */
+	public function test_cloudinary_src_is_tracked_back_to_its_attachment() {
+		$delivery = $this->get_delivery();
+		$url      = $this->seed_known( $delivery );
+
+		$element = $delivery->parse_element( '<img width="300" height="225" src="' . self::BASE_URL . '/images/w_300,h_225,c_scale/f_auto,q_auto/v1/canola/canola.jpg?_i=AA" class="attachment-medium size-medium" alt="" />' );
+
+		$this->assertSame( self::$attachment_id, $element['id'] );
+		$this->assertSame( $url, $element['atts']['src'] );
+		$this->assertSame( 'canola.jpg', $element['atts']['data-public-id'] );
+	}
+
+	/**
+	 * A Cloudinary src with the plugin's own thumbnail crop is still tracked back.
+	 *
+	 * @return void
+	 */
+	public function test_cloudinary_src_with_thumbnail_crop_is_tracked_back() {
+		$delivery = $this->get_delivery();
+		$this->seed_known( $delivery );
+
+		$element = $delivery->parse_element( '<img width="150" height="150" src="' . self::BASE_URL . '/images/w_150,h_150,c_thumb,g_auto/f_auto,q_auto/v1/canola/canola.jpg" alt="" />' );
+
+		$this->assertSame( self::$attachment_id, $element['id'] );
+	}
+
+	/**
+	 * A Cloudinary src with transformations of its own, e.g. typed into content by hand, is left
+	 * alone, since rebuilding it from the attachment would drop them.
+	 *
+	 * @dataProvider data_custom_transformations
+	 *
+	 * @param string $transformations The transformations in the src.
+	 *
+	 * @return void
+	 */
+	public function test_cloudinary_src_with_custom_transformations_is_not_tracked_back( $transformations ) {
+		$delivery = $this->get_delivery();
+		$this->seed_known( $delivery );
+
+		$src     = self::BASE_URL . '/images/' . $transformations . '/v1/canola/canola.jpg';
+		$element = $delivery->parse_element( '<img src="' . $src . '" alt="" />' );
+
+		$this->assertSame( 0, $element['id'] );
+		$this->assertSame( $src, $element['atts']['src'] );
+	}
+
+	/**
+	 * Transformations the plugin doesn't apply itself.
+	 *
+	 * @return array
+	 */
+	public function data_custom_transformations() {
+		return array(
+			'effect'      => array( 'w_300,h_225,c_scale/e_sepia/f_auto,q_auto' ),
+			'manual crop' => array( 'c_crop,w_400,h_400,x_100,y_100/e_grayscale' ),
+			'gravity'     => array( 'w_300,h_300,c_fill,g_face' ),
+		);
+	}
+
+	/**
+	 * A Cloudinary src for a public ID Delivery doesn't know is left alone.
+	 *
+	 * @return void
+	 */
+	public function test_unknown_cloudinary_src_is_not_tracked_back() {
+		$delivery = $this->get_delivery();
+		$this->seed_known( $delivery );
+
+		$src     = self::BASE_URL . '/images/w_300,h_225,c_scale/v1/other/other.jpg';
+		$element = $delivery->parse_element( '<img width="300" height="225" src="' . $src . '" alt="" />' );
+
+		$this->assertSame( 0, $element['id'] );
+		$this->assertSame( $src, $element['atts']['src'] );
+	}
+}
+
+/**
+ * Media with cloudinary_url() stubbed, to avoid the real sync/signature pipeline.
+ */
+class Test_Delivery_Parse_Element_Media extends \Cloudinary\Media {
+
+	/**
+	 * Canned cloudinary_url() return value.
+	 *
+	 * @var string|false
+	 */
+	public $stub_cloudinary_url = false;
+
+	/**
+	 * Stubbed to avoid the real sync/signature pipeline.
+	 *
+	 * @param int          $attachment_id             The attachment ID.
+	 * @param array|string $size                      The requested size.
+	 * @param array        $transformations           Transformations to apply.
+	 * @param string|null  $cloudinary_id             A forced Cloudinary ID.
+	 * @param bool         $overwrite_transformations Whether to overwrite transformations.
+	 *
+	 * @return string|false
+	 */
+	public function cloudinary_url( $attachment_id, $size = array(), $transformations = array(), $cloudinary_id = null, $overwrite_transformations = false ) {
+		return $this->stub_cloudinary_url;
+	}
+}
+
+/**
+ * Delivery with a settable media component, since init() only wires the real one once connected.
+ */
+class Test_Delivery_Parse_Element_Delivery extends \Cloudinary\Delivery {
+
+	/**
+	 * Set the media component.
+	 *
+	 * @param \Cloudinary\Media $media The media component.
+	 *
+	 * @return void
+	 */
+	public function set_media( $media ) {
+		$this->media = $media;
+	}
+}