I found something very nice in the System Preferences today, specifically in the Desktop prefs panel. I wanted to change the desktop image to a photo I shot some time ago. What I did not remember was that this image contained a cropping path, because it's original had been printed that way.
Now when I selected the new image, I could not believe my eyes: my desktop still showed my old background, plus the perfectly cropped new one overlayed upon it! This configuration survived even a logout and login procedure, so I think it's stable.
Obviously curious, I tried to put a second cropped image over the other two, and believe it or not, it worked again! Now i happily had three images coexisting on my desktop, one placed neatly over the other - like stickers.
[Editor's note: I don't have access to a program capable of creating cropping paths, so I can't test this one myself. However, luc did send me screenshots showing the two full-sized images in his Desktop selection area with the actual desktop showing clipped versions of both images, so it certainly appears to be true!]
Mac OS X Hints