you could add an input field to allow people to enter an imagr url, but i didn't do that. you would wind up with dead img links from old members.
they way my custom icon works, follows.
usernumber 00000004 wants a custom icon that ony he can use. you name his avatar 000000004.gif and upload it to the custom dir. then when usernumber 00000004 goes to edit his profile, there is a note alerting him that his custom avatar is available and there is an extra choice in the dropdown called 'custom'.
thought about using an img upload script that would allow you to up your own. auto chose the custum dir and auto rename the file usernumber.gif. that way you could up the img and refgresh the modify page and the custom icon would be avail. problem is I have found a good way to limit file size and I'm not interested in people uploading huge files. if anybody has a clean upload script that lets you set file type and size restrictions as well as set the upload dir, let me know and I'll add it as a cp option.
