I started a whole new thread with related image optimization ideas until your relevancy engine led me here.
If you combined this (or some RESS solution) with some back end processing to automate two of the services (Sprites/TinyPNG) that @bartekkustra pointed to in [Optimization] Tips and tricks - #2 by thesergie - Webflow Tips - Forum | Webflow, it could be an unbeatable combo:
Wouldn’t it be awesome if all the images in a site or page were automatically turned into a sprite sheet, and then post-processed with the excellent TinyPNG algorithm? With one sheet per breakpoint served up in this manner, you’d have made screaming image performance a seamless UX no-brainer.
A checkbox option to not include an image in the sprite sheet might be useful if a site or page was particularly graphic intensive, so hero images at the top could load with priority over the sprites.