ok, what you need to do is find a pic that you want to use. then you gotta use a photo editing program like photoshop or paintshop pro, etc. to shrink the size to 100x100. then you click on user cp at the top of the page, then click on avatar. under the smiley faces there is a section for custom avatars. just browse your computer until youve found it and then upload it to UM. or if it is small enough on the web, you can type in the URL and bam!, avatar.
about changing your avatars; go to your "user CP" (look left-top of the screen), then go to the "avatar section. and there on the bottom of that site you can upload your avatar or add a link if you have it somewhere on the web.
hope I helped you...
have a nice time here!
EDIT: yetti!! grr..ah..better two answers than none