I don't see any reason why 1080p shouldn't be allowed, it won't take that much place at all. I took a random picture for a comparison and saved it in multiple ways.
http://puu.sh/jlgTR/eb346fe7f7.png (5.69 MB, PNG 2560x1440) - Original, forbidden to use
http://puu.sh/jlgAI/066cb99a07.jpg (1.28 MB, JPG 90% quality, 2560x1440) - Forbidden to use
http://puu.sh/jlgTe/71b3126984.jpg (2.08 MB, JPG 100% quality, 2560x1440) - Forbidden to use
http://puu.sh/jlgoM/80f186fb3f.png (2.65 MB, PNG 1920x1080) - Forbidden to use
http://puu.sh/jlglR/a02b66d1b1.jpg (778 kB, JPG 90% quality, 1920x1080) - Forbidden to use
http://puu.sh/jlgKQ/e47e4eb700.jpg (1.12 MB, JPG 100% quality, 1920x1080) - Forbidden to use
http://puu.sh/jlh20/4e17ff554f.png (1.34 MB, PNG 1366x768) - Allowed to use, frequently used
http://puu.sh/jlgtg/9be5d7bdb9.jpg (454 kB, JPG 90% quality, 1366x768) - Allowed to use
http://puu.sh/jlgQf/ec408c7a73.jpg (648 kB, JPG 100% quality, 1366x768) - Allowed to use, frequently used
What's problem here is that beatmaps are getting ranked with PNGs and 100% quality JPGs and are pretty frequently used. However nobody is considering that 90% quality doesn't make any difference for human's eye, compare those 90% images to PNGs and 100% images, I doubt you can notice a big difference, if any. People use these pngs in storyboards and even for regular backgrounds, but if it doesn't matter why would 90% JPG with 2560x1440 resolution matter and why does actually 90% JPG with 1920x1080 quality matter when it does have lower size than 1366x768 PNG and almost equal size to 100% JPG with 1366x768 resolution. I think if people care about the fact that we can use a bit lower quality, receive lower file size but still not notice anything, it shouldn't be problem at all.