diff --git a/src/ckeditor/image/ImageDowncastPlugin.ts b/src/ckeditor/image/ImageDowncastPlugin.ts
new file mode 100644
index 0000000000..e8448899dd
--- /dev/null
+++ b/src/ckeditor/image/ImageDowncastPlugin.ts
@@ -0,0 +1,84 @@
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+import type { ViewDocumentFragment, ViewElement } from 'ckeditor5'
+
+import { ImageUtils, Plugin, UpcastWriter } from 'ckeditor5'
+
+/**
+ * Parse a CSS length into whole pixels. Anything but an absolute pixel value
+ * yields null.
+ *
+ * @param value the CSS length to parse
+ */
+function toPixels(value: string | undefined): number | null {
+	if (value === undefined) {
+		return null
+	}
+
+	const pixels = /^\s*([\d.]+)\s*px\s*$/.exec(value)
+	if (pixels === null) {
+		return null
+	}
+
+	const width = Math.round(Number.parseFloat(pixels[1]))
+
+	return width > 0 ? width : null
+}
+
+/**
+ * Sizes images in the editor's data output only; the editing view keeps
+ * CKEditor's own markup.
+ */
+export default class ImageDowncastPlugin extends Plugin {
+	static get requires() {
+		return [ImageUtils] as const
+	}
+
+	static get pluginName() {
+		return 'ImageDowncast' as const
+	}
+
+	init(): void {
+		// The width and the natural size are written by two separate converters,
+		// so post-process the finished view instead of overriding either.
+		this.editor.data.on('toView', (event) => {
+			const fragment = event.return as ViewDocumentFragment
+			const writer = new UpcastWriter(fragment.document)
+
+			for (const { item } of writer.createRangeIn(fragment)) {
+				// A block image carries the resized width on its figure, an inline
+				// one on the img itself.
+				if ((item.is('element', 'figure') && item.hasClass('image')) || item.is('element', 'img')) {
+					this._mirrorResizedWidth(writer, item)
+				}
+			}
+		}, { priority: 'low' })
+	}
+
+	/**
+	 * Mirrors a resized image's CSS width onto the img width attribute, which
+	 * clients that drop CSS still honour. Reopening the message reads that width
+	 * back as the image's natural size.
+	 *
+	 * @param writer view writer of the data view
+	 * @param element the figure or img holding the resized width
+	 */
+	_mirrorResizedWidth(writer: UpcastWriter, element: ViewElement): void {
+		const resizedWidth = toPixels(element.getStyle('width'))
+		if (resizedWidth === null) {
+			return
+		}
+
+		const image = this.editor.plugins.get('ImageUtils').findViewImgElement(element)
+		if (image === undefined) {
+			return
+		}
+
+		writer.setAttribute('width', String(resizedWidth), image)
+		// A natural height next to the smaller width would stretch the image.
+		writer.removeAttribute('height', image)
+	}
+}
diff --git a/src/components/TextEditor.vue b/src/components/TextEditor.vue
index 53b9ca4ee2..3b9a54b139 100644
--- a/src/components/TextEditor.vue
+++ b/src/components/TextEditor.vue
@@ -55,6 +55,7 @@ import {
 import { getLinkWithPicker, searchProvider } from '@nextcloud/vue/components/NcRichText'
 import TextDirectionPlugin from '../ckeditor/direction/TextDirectionPlugin.js'
 import FilesImagePlugin from '../ckeditor/image/FilesImagePlugin.ts'
+import ImageDowncastPlugin from '../ckeditor/image/ImageDowncastPlugin.ts'
 import MailPlugin from '../ckeditor/mail/MailPlugin.js'
 import QuotePlugin from '../ckeditor/quote/QuotePlugin.js'
 import SignaturePlugin from '../ckeditor/signature/SignaturePlugin.js'
@@ -152,6 +153,7 @@ export default {
 				ImageUpload,
 				ImageResize,
 				FilesImagePlugin,
+				ImageDowncastPlugin,
 				Font,
 				RemoveFormat,
 				Base64UploadAdapter,
@@ -201,6 +203,11 @@ export default {
 				plugins,
 				toolbar,
 				language: 'en',
+				image: {
+					// A percentage would be relative to the recipient's unknown viewport.
+					resizeUnit: 'px',
+				},
+
 				mention: {
 					feeds: [
 						{
diff --git a/src/tests/unit/ckeditor/image/ImageDowncastPlugin.spec.js b/src/tests/unit/ckeditor/image/ImageDowncastPlugin.spec.js
new file mode 100644
index 0000000000..1d5f51908a
--- /dev/null
+++ b/src/tests/unit/ckeditor/image/ImageDowncastPlugin.spec.js
@@ -0,0 +1,112 @@
+/**
+ * SPDX-FileCopyrightText: 2026 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+import { ClassicEditor, ImageBlock, ImageInline, ImageResizeEditing, Paragraph } from 'ckeditor5'
+import ImageDowncastPlugin from '../../../../ckeditor/image/ImageDowncastPlugin.ts'
+
+// The editor UI observes the size of its toolbar, which jsdom does not provide.
+window.ResizeObserver = class {
+	observe() {}
+	unobserve() {}
+	disconnect() {}
+}
+
+/**
+ * Get the data of an editor initialised with the given content.
+ *
+ * @param {string} initialData content to load into the editor
+ * @return {Promise<string>} the editor's data output
+ */
+async function downcast(initialData) {
+	const element = document.createElement('div')
+	document.body.appendChild(element)
+
+	const editor = await ClassicEditor.create(element, {
+		licenseKey: 'GPL',
+		initialData,
+		plugins: [Paragraph, ImageBlock, ImageInline, ImageResizeEditing, ImageDowncastPlugin],
+		image: {
+			resizeUnit: 'px',
+		},
+	})
+
+	const data = editor.data.get()
+	await editor.destroy()
+	element.remove()
+
+	return data
+}
+
+/**
+ * A 400x300 image resized to the given CSS width.
+ *
+ * @param {string} width the CSS width on the figure
+ * @return {string} the figure markup
+ */
+function resized(width) {
+	return `<figure class="image image_resized" style="width:${width};">`
+		+ '<img src="test.png" width="400" height="300">'
+		+ '</figure>'
+}
+
+describe('ImageDowncastPlugin', () => {
+	it('mirrors a resized width onto the width attribute', async () => {
+		const data = await downcast(resized('200px'))
+
+		expect(data).toContain('width="200"')
+		expect(data).toContain('width:200px;')
+	})
+
+	it('drops the natural height so the image is not stretched', async () => {
+		const data = await downcast(resized('200px'))
+
+		expect(data).not.toContain('height=')
+	})
+
+	it('keeps the aspect ratio the editor writes', async () => {
+		const data = await downcast(resized('200px'))
+
+		expect(data).toContain('aspect-ratio:400/300;')
+	})
+
+	it('keeps the attributes of an image resized to a percentage', async () => {
+		const data = await downcast(resized('50%'))
+
+		expect(data).toContain('width="400"')
+		expect(data).toContain('height="300"')
+	})
+
+	it('keeps the attributes of an image that was not resized', async () => {
+		const data = await downcast('<figure class="image"><img src="test.png" width="400" height="300"></figure>')
+
+		expect(data).toContain('width="400"')
+		expect(data).toContain('height="300"')
+	})
+
+	it('mirrors the width of a linked image', async () => {
+		const data = await downcast('<figure class="image image_resized" style="width:100px;">'
+			+ '<a href="https://nextcloud.com"><img src="test.png" width="400" height="300"></a>'
+			+ '</figure>')
+
+		expect(data).toContain('width="100"')
+		expect(data).not.toContain('height=')
+	})
+
+	it('mirrors the width of a resized inline image', async () => {
+		const data = await downcast('<p>text <img class="image_resized" style="width:200px;" src="test.png" width="400" height="300"></p>')
+
+		expect(data).toContain('width="200"')
+		expect(data).not.toContain('height=')
+	})
+
+	it('keeps the width when the message is reopened and sent again', async () => {
+		const sent = await downcast(resized('200px'))
+		const resent = await downcast(sent)
+
+		expect(resent).toContain('width="200"')
+		expect(resent).toContain('width:200px;')
+		expect(resent).not.toContain('height=')
+	})
+})
diff --git a/src/tests/unit/components/TextEditor.spec.js b/src/tests/unit/components/TextEditor.spec.js
index 7930c8e51a..eb8c6b89a1 100644
--- a/src/tests/unit/components/TextEditor.spec.js
+++ b/src/tests/unit/components/TextEditor.spec.js
@@ -8,6 +8,7 @@ import { GeneralHtmlSupport, Paragraph } from 'ckeditor5'
 import mitt from 'mitt'
 import { vi } from 'vitest'
 import TextEditor from '../../../components/TextEditor.vue'
+import ImageDowncastPlugin from '../../../ckeditor/image/ImageDowncastPlugin.ts'
 import MailPlugin from '../../../ckeditor/mail/MailPlugin.js'
 import Nextcloud from '../../../mixins/Nextcloud.js'
 import VirtualTestEditor from '../../virtualtesteditor.js'
@@ -56,6 +57,23 @@ describe('TextEditor', () => {
 		expect(wrapper.vm.config.htmlSupport.allow.some((rule) => rule.name === 'img')).toBe(true)
 	})
 
+	it('resizes images in pixels in html mode', async () => {
+		const wrapper = shallowMount(TextEditor, {
+			localVue,
+			provide: {
+				addToFocusTrap: vi.fn(),
+			},
+			propsData: {
+				value: 'bonjour',
+				html: true,
+				bus: mitt(),
+			},
+		})
+
+		expect(wrapper.vm.config.plugins).toContain(ImageDowncastPlugin)
+		expect(wrapper.vm.config.image.resizeUnit).toBe('px')
+	})
+
 	it('throw when editor not ready', async () => {
 		const wrapper = shallowMount(TextEditor, {
 			localVue,