Go to User CP
http://img163.imageshack.us/img163/9752/usercp.gif
Scroll down to Settings & Options, click on Edit Avatar
http://img197.imageshack.us/img197/8091/editavy.gif
Go to the Custom Avatar Section
http://img339.imageshack.us/img339/2545/customavy.gif
You can either link an avatar or upload one from your files
Do not forget to save changes
http://img811.imageshack.us/img811/4194/savechanges.gif