The Darkness writhes!
It's actually pretty simple Mrs. Hollow.
Let's use my recent work for example: Simply upload and arrange the order you'd like:

You could also use Paint, or Picasa or any other free programs to crop any photo's you'd like to use, getting something akin to this:

Then, once those free sites have uploaded it, simply right click for the images URL.
For example:
http://i.imgflip.com/2nf2u.gifYou would use that in your Avatar Settings.