Go to your settings, just fill in the URL for remote avatar (you can for example upload an avatar on photobucket).
Or you can upload it on this site, just by clicking on the browse button next to "upload you avatar".
Maximum avatar size (wxh) is 120 pixels. x 100 pixels. So be sure your avatar isn't larger than that!
For signature: go to settings, signature and then fill in the url of your signature (which you've upload on for example photobucket).
something like this:
[img]http://photobucket.com/mysignature[/img]
