This is desperately needed for high-quality maps that run on low-end computers. I like to enhance the detail of my maps with lots of tokens, but that puts a heavy strain on players' computers that have load a bunch of different overlapping images. For instance, if I put a floor on top of a high-res image of a room, and then things on that floor, there's suddenly three different layers of shenanigans to load on the map layer alone. A flatten image functionality isn't as necessary on small maps, but on large maps, especially dungeons, Roll20 can't really do much to help the players unless you stitch together printscreened images like a user suggested above.