well, now I don't get the url message... however, it is telling me that it's not a valid jpg or gif file... the url ends in ".gif", so shouldn't it work?
I would guess that it doesn't work because the link is to a file on your mail server which requires a username and password. You might try what Jordan is suggesting. Save it to your desktop and upload it to GTPlanet with the "browse" option in the Avatar section.
Viper: All of your questions can be answered in the FAQ at the top of the page. However, I'm a nice guy, so I'll answer your questions right here.
1) You can get your own avater after you have 200 posts. If you want to make your own, click on any avater for a little pop-up that'll tell you how to.
2) As you'll read in the FAQ, you can not change your username.