Try linking to an off-site (well, not really off-site here, but that shouldn't be the problem probably...) avatar using a server's IP address
URL:

http://192.168.2.7/~username/forum/styles/prosilver/imageset/site_logo.gif

=> "The URL you specified is invalid."

The "problem" seems to be the *\.[a-z]

if (!preg_match('#^(http|https|ftp)://(.?\.)?[a-z0-9\-]+?\.[a-z]{2,4}

:?([0-9]?).?\.(gif|jpg|jpeg|png)$#i', $data['remotelink']))

I'm no regex expert, but removing it helped, i.e. that should be optional or so if possible.
Is that a bug or intended (i.e. "We don't allow linking to IP-only servers")?