I made an aquarium, because it was the most awesome thing I could think of.
I had a lot of art noir and illustrations laying around, and I like sea creatures. It was a natural fit. I used the magic wand tool to take the bulk of the backgrounds out, and used the eraser tool with smooth edging for the details. This took me about 7 hours, and I just sat down and did the entire thing. The first panel took the longest, but once it was done, the rest came easily. Each panel has something a bit different going on. See if you can find them all.