The guest user icon is a PNG file stored deep within OS X. I don't recommend you change operating system components that exist at that level though. Besides, the change is not likely to survive an OS X update.
If you really want to change it I'll post its location, but you'll be on your own if something breaks.
Edit: You'd be facing the same problem as in your other question: How to change folder symbol on desktop back to original? Modifying the Mac's basic user interface fundamentals can result in instability. If you want to perform that sort of customization anyway, always have a backup strategy in case something goes wrong. To learn how to back up and restore read Mac Basics: Time Machine backs up your Mac.