-Dimensions, anything square. The site will resize it through html. 100x100 is the most commonly used size.
-Upload your image to a hosting site. (tinypic.com, imageshack.us, photobucket.com, and so on)
Save the direct link to that image.
Browse to your character profile. (character in the right hand menu, followed by edit [name] profile)
Replace the default character url with the one of your uploaded image.