Any AIM buddy icon site will work. Just don't get an animated one, iChat won't show anything except the first fram of the animation.
To use one you find on a site, drag it (or save it) to your desktop, then click your picture on your iChat buddy list and go to edit picture. Drag the image file from your desktop onto the designated area and use the slider to set the size and drag the image around to set the position.
Oh, and about the icon thing, I got it worked out.. thx guys. I ended up taking a pic of my school logo from a bag i got from my school, and now i use that.