My first map to use a custom background is here. It took a lot of time to cut everything out and then add new areas to the level...
Bionic Commando - Area 7 by EastX
Looking forward to that new Tetris map, SEH! I have an idea for one too, but I am holding off on it due to your getting there first. Hydro, are you going to dip your toes into the custom map pool sometime?