Thank you! I got it working now.
The exactly script I'm using is:
$avatardir = "http://www.wiistation.nl/forum/img/avatars";
if ($img_size = @getimagesize($avatardir.'/'.$list2->userid.'.gif'))
$user_avatar = '<img src="'.$avatardir.'/'.$list2->userid.'.gif" alt="" />';
else if ($img_size = @getimagesize($avatardir.'/'.$list2->userid.'.jpg'))
$user_avatar = '<img src="'.$avatardir.'/'.$list2->userid.'.jpg" alt="" />';
else if ($img_size = @getimagesize($avatardir.'/'.$list2->userid.'.png'))
$user_avatar = '<img src="'.$avatardir.'/'.$list2->userid.'.png" alt="" />';