Once I played a similar prank to a computer science teacher. Back in the Windows 3.x for Workgroups era this was. I made a screenshot of the desktop (showing a window), and put it on as wallpaper. Took the man a little while to figure out why that window couldn't be closed (after a hard reboot later when the window popped back up :) )

You might enjoy this GDC talk by Mark Ferrari of LucasArts fame, where he goes over his pixel art technique, as well as how he did color cycling: https://www.youtube.com/watch?v=aMcJ1Jvtef0