It may be nice to have it as a feature via the UI or something similar on another level under the page. Have it so the user can chose and set a mockup / design image, the ability to toggle it on and off, set the opacity or overlay color, and delete the image as well.
Just an idea, see what @matjaz thinks. 
I also like the idea of a grid system overlay (like in the older thread) to toggle on and off via the UI.