That would be an 'Avatar'. Basically, any image thats no more that 150x150 pixels in size. If you go to your Profile page and scroll all the way to the bottom there is a box labelled 'Avatar Control'. There is a box there to input the web address for the image you want to use (it has to be hosted someplace else on the net, like cardomain.com or one of the other free sites). If you need more info or help PM me.
Oh, don't forget that for a flat $10 donation you can have an animated avatar made and hosted on the LOST server, or for $5 you can have a normal avatar image hosted (Figured I'd better put that little plug in there for donations
).