Find a picture you like on the internet and right click on the url and select copy. (For example, my av is located at:
http://mysite.verizon.net/TheAvengers/avanimate.gif ).
Then go to the WoKJ forums, click on profile and scroll down to "Link to off-site Avatar:" and paste that url in the box there.
Doing it that way you won't have to worry about the size, but if it is too big it will get chopped off, so I'd suggest a smaller size picture.
Hope that helps.