Kaz,
Creating avatars like Rachel's is easy if you get hold of a thumbnail generator. Do a web search for "thumbnail generator" and you'll find a few. That's how I got mine - its a thumbnail made from a full sized pic.
Thumbnail generators do much more than just scaling the picture, like you can do with image editors. They use compression routines to retain clarity. You can also specify the exact size you want, so you can meet Adrian's standards for this site.
Have fun!
James.