13 and a linux user, that's cool. The gimp is pretty powerful. I have a usb stick with knoppix and I use the gimp when I don't have access to photoshop.
It should be fine for sigs.
Edit: Just realized there is now a win32 version of the gimp.
You can always right click and go to properties if you're still using IE.
Otherwise the code for the avatar is:
http://img213.exs.cx/img213/571/nym91avatar3nt.jpg
In the downloads section, there are 4 in all.
KC's psds, Umachines jersey template, Rolies 2X White, and Thome25's throwback template.
Hope that helps.