I just did a web search for 48X48 icons and found this one. http://www.w3.org/Icons/48x48/smiley.gif
I then pasted the link into the Use Custum URL box of the Avatar selection screen. Lets see if it worked...
Edit: changed to http://horgad.freewebspace.com/images/cuda.jpg
1. signed up for some free web space
2. found a picture I liked on this site
3. pasted the picture into an editor and cut and shrunk until I had a 48X48 picture
4. uploaded the picture to my free website