you would set that in your admin control panel
"Profile Settings" and the "Avatars" tab
make sure the path ( disk based location ) is writeable and that the URL ( web based location ) are consistent.
for Mindraven, they use WHM / CPanel, so the path would be something like:
/home/yourlogin/public_html/yourforum/images/avatars
and the url would be:
http://yourforum/images/avatars