its called a signature, sig for short. they like myself make them in programs like photoshop, paintshop pro, any image editing programs.
and if you ever get around to making one or having someone else make one, you have to upload it to an image uploader that some sites have on them. then you apply the address of the image that you have uploaded, in the sig box in your user cp/profile.
if you have further questions ask about it in the art forum. you will get the most help there.