In the meantime...

...Secure a high quality JPG/PNG file and drop onto your design page.

Scale the image to your wanted size.
Use an on-line image to webP converter - I tried https://convertio.co/webp-converter/ as it seemed to be the best out there.
Save the converted webP file in the same folder as your original (just for convenience).
Select the image in your design and press Shift+Ctrl+Alt+A and add the full path to your webP file.
Finally, open the same selected image's Placeholder code body and add: <img src="index_htm_files/[image].webp" width=100% height=100% />, where [image] is your filename.
Do not have Regenerate placeholder image ticked and click out with the OK button a few times.

Your XDA will now use the webP file instead of your JPEG/PNG.

Far from perfect but it does work: https://shared.xara.com/ftJWjAASDx/.

In return, I need the code that would handle the stretched placeholder image in a similar fashion to Stretch Full Width.

Acorn