How to modify guest account properties?

A few days ago, my brother was using the guest account on my computer, took a few photos, checked his email, and didn’t seem to do anything malicious, but when I next logged on, the picture for the guest account was changed.

This is unusual to me because the guest account deletes all information upon logout, including the picture.
I’ve tried disabling the guest account, restarting, all the usual stuff, but the picture remains.

It’s not that big of a deal, but something that I don’t necessarily think should be happening. Running OSX 10.6.2.