That would add unnecessary data overhead as clients would get image data they may not need, e.g. a low-resolution device would not want a high-DPI version of all images on the site.
> A subset video bitstream is derived by dropping packets from the larger video to reduce the bandwidth required for the subset bitstream. The subset bitstream can represent a lower spatial resolution (smaller screen), lower temporal resolution (lower frame rate), or lower quality video signal.
Inventing a new protocol if the problem can be solved by serving the proper markup, seems misguided.