I didn't get what you want exactly, is it just to resize the file to the right size?
If you're getting an error when you try uploading it here use a image storage site like ImageShack and put the URL there instead of uploading it. I had to do that because I kept getting errors when I tried to upload.
Hope it helped you '
zeena, take the avatar you want, place it in paint or photoshop, resize it to 24x24, post it in an album in photobucket (or some other photo place), use the URL that photoshop gives you when you click change avatar.