getr any picture you want, and crop it (cut out the part you wanna use) or resize the whole picture in any photo editor. or you can do wat i do and create your own
as for uploading it, you go your control panel (small white letters on top right corner) and choose the "user options" tab. go to the bottom and click "change avatar". the rest is self explanitory. remember that they haev to be at the most 65 X 65 pixels.