There are few of them on these forums, but I just found out that you can use any small image! First select the Perspective you want to affect. Then click once on the large icon at the bottom that corresponds to it. (The same one you double-click to get the pop-up collection.)
You can then paste any small image from the clipboard and - bam! - it appears as the icon. If you have Skitch
installed, you can grab a screenshot of anything, size it to a small square (I did 58x58 pixels), then just drag it onto the icon at the bottom of the window, the same way you'd drag it to your desktop.
Works like a charm!