The thing is, I can only work in screen modes that the user is running. So if I made an NTSC 320 x 400 mode, the user would have to have loaded the NTSC monitor driver, or it wont work. Same with productivity modes and RTGs, and I have no way to know what the user would be running. As a compromise, I could simply have it running as a Workbench App, so it will run in whatever screen mode and colours is on your workbench, and it should run like that.
Another issue is the app is only capable of editing the first 256 colours. It can load and save any number, but there is no space on the display to make an edit box for 4096, 16.8M colours etc.
I am avoiding looking at Datatypes and file-type compatibility for now, so I cant say yes it will edit 5 megabyte PNGs in 1080p.