diff --git a/addons/web_editor/static/src/js/editor/image_processing.js b/addons/web_editor/static/src/js/editor/image_processing.js
index 1308c23b0e0..ee52f6ad31f 100644
--- a/addons/web_editor/static/src/js/editor/image_processing.js
+++ b/addons/web_editor/static/src/js/editor/image_processing.js
@@ -233,6 +233,8 @@ export async function applyModifications(img, dataOptions = {}) {
     // Crop
     const container = document.createElement('div');
     const original = await loadImage(originalSrc);
+    // loadImage may have ended up loading a different src (see: LOAD_IMAGE_404)
+    originalSrc = original.getAttribute('src');
     container.appendChild(original);
     await activateCropper(original, 0, data);
     let croppedImg = $(original).cropper('getCroppedCanvas', {width, height});
@@ -358,6 +360,7 @@ export function loadImage(src, img = new Image()) {
         img.src = source;
     };
     // The server will return a placeholder image with the following src.
+    // grep: LOAD_IMAGE_404
     const placeholderHref = "/web/image/__odoo__unknown__src__/";
 
     return new Promise((resolve, reject) => {
@@ -419,6 +422,8 @@ async function _updateImageData(src, key = 'objectURL') {
 }
 /**
  * Returns the size of a cached image.
+ * Warning: this supposes that the image is already in the cache, i.e. that
+ * _updateImageData was called before.
  *
  * @param {String} src used as a key on the image cache map.
  * @returns {Number} size of the image in bytes.
diff --git a/addons/website/static/tests/tours/drop_404_ir_attachment_url.js b/addons/website/static/tests/tours/drop_404_ir_attachment_url.js
new file mode 100644
index 00000000000..3906ac20d7c
--- /dev/null
+++ b/addons/website/static/tests/tours/drop_404_ir_attachment_url.js
@@ -0,0 +1,44 @@
+/** @odoo-module **/
+
+import wTourUtils from '@website/js/tours/tour_utils';
+
+wTourUtils.registerWebsitePreviewTour('drop_404_ir_attachment_url', {
+    test: true,
+    url: '/',
+    edition: true,
+}, () => [
+    wTourUtils.dragNDrop({
+        id: 's_404_snippet',
+        name: '404 Snippet',
+    }),
+    {
+        content: 'Click on the snippet image',
+        trigger: 'iframe .s_404_snippet img',
+    }, {
+        content: 'Once the image UI appears, check the image has no size (404)',
+        trigger: 'iframe .s_404_snippet img',
+        extra_trigger: '.snippet-option-ReplaceMedia',
+        run: function () {
+            const imgEl = this.$anchor[0];
+            if (!imgEl.complete
+                    || imgEl.naturalWidth !== 0
+                    || imgEl.naturalHeight !== 0) {
+                console.error('This is supposed to be a 404 image');
+            }
+        },
+    },
+    wTourUtils.changeOption('ImageTools', 'we-select[data-name="shape_img_opt"] we-toggler'),
+    wTourUtils.changeOption('ImageTools', 'we-button[data-set-img-shape]'),
+    {
+        content: 'Once the shape is applied, check the image has now a size (placeholder image)',
+        trigger: 'iframe .s_404_snippet img[src^="data:"]',
+        run: function () {
+            const imgEl = this.$anchor[0];
+            if (!imgEl.complete
+                    || imgEl.naturalWidth === 0
+                    || imgEl.naturalHeight === 0) {
+                console.error('Even though the original image was a 404, the option should have been applied on the placeholder image');
+            }
+        },
+    },
+]);
diff --git a/addons/website/tests/test_ui.py b/addons/website/tests/test_ui.py
index 92ef59e4edf..fd967304945 100644
--- a/addons/website/tests/test_ui.py
+++ b/addons/website/tests/test_ui.py
@@ -515,3 +515,40 @@ class TestUi(odoo.tests.HttpCase):
             'path': 'website/static/tests/tour_utils/widget_lifecycle_patch_wysiwyg.js',
         })
         self.start_tour(self.env['website'].get_client_action_url('/'), 'widget_lifecycle', login='admin')
+
+    def test_drop_404_ir_attachment_url(self):
+        website_snippets = self.env.ref('website.snippets')
+        self.env['ir.ui.view'].create([{
+            'name': '404 Snippet',
+            'type': 'qweb',
+            'key': 'website.s_404_snippet',
+            'arch': """
+                <section class="s_404_snippet">
+                    <div class="container">
+                        <img class="img-responsive img-thumbnail" src="/web/image/website.404_ir_attachment"/>
+                    </div>
+                </section>
+            """,
+        }, {
+            'type': 'qweb',
+            'inherit_id': website_snippets.id,
+            'arch': """
+                <xpath expr="//t[@t-snippet='website.s_parallax']" position="after">
+                    <t t-snippet="website.s_404_snippet"
+                       t-thumbnail="/website/static/src/img/snippets_thumbs/s_website_form.svg"/>
+                </xpath>
+            """,
+        }])
+        attachment = self.env['ir.attachment'].create({
+            'name': '404_ir_attachment',
+            'type': 'url',
+            'url': '/web/static/__some__typo__.png',
+            'mimetype': 'image/png',
+        })
+        self.env['ir.model.data'].create({
+            'name': '404_ir_attachment',
+            'module': 'website',
+            'model': 'ir.attachment',
+            'res_id': attachment.id,
+        })
+        self.start_tour(self.env['website'].get_client_action_url('/'), 'drop_404_ir_attachment_url', login='admin')