If you don't have a graphics application:
http://www.resize2mail.com/advanced.php
Hints:
To keep filesize at a minimum, answer the following:
1. Is the image a PICTURE, i.e. a photograph, or is it GRAPHICAL, i.e. a cartoon
If a picture, choose JPG. If Graphical, choose gif.
2. If quality is important, i.e. definition (is there fine lines, small text etc) then choose GIF, otherwise, choose JPG
3. Is the avatar animated?
If YES... choose GIF. JPEG files won't animate. GIF files will.