Just go to a site you like that has pics you like. If they are too big, y ou can usually ask someone here to re-size it for you if you can't.
This can be found in the " How To " section on the BB.
A lot of mine are thumbnails that are exactly the right size after I steal them from where ever they are put up at. My current one I resized myself with Photoshop and another program.
That's what I usually do SilverVeil. My dad and brother recently joined a message board and were looking for avatars and well, the kind I usually use aren't what they want. *shrugs*