I'm not entirely sure what you're specifically asking, so I'll go through it all.
To change your avatar, go to your
profile and scroll to the bottom.
Make sure your avatar's no larger than 80x80 pixels, no more than 25 kb, and either GIF, JPG, or PNG. BMP and other formats such as PHP or ASP
don't work, unless you're using it in your signature. Also, it must be uploaded onto a site, not brought directly from your computer.
Imageshack's a good one you can use, you don't need to register.
After you've retrieved the URL in which Imageshack (or whatever host you're using) gives you, you can then use it for PPT.