Yea, I already implemented something similar at our boards. We are just using the noavatar.gif.
Here's a snippit of the code for admin/functions.php, although, you should edit private.php and announcement.php as well.
PHP Code:
if ($post[avatarid]!=0) {
$avatarurl=$post[avatarpath];
} else {
if ($post[hascustomavatar] and $avatarenabled) {
$avatarurl="avatar.php?userid=$post[userid]&dateline=$post[avatardateline]";
} else {
$avatarurl="images/avatars/noavatar.gif";
}
}
if ($bbuserinfo[userid]>0 and !($bbuserinfo[showavatars])) {
$post[avatar]="";
} else {
if ($post['lastactivity'] > $datecut and !$post['invisible'] and $post['lastvisit'] != $post['lastactivity']) {
eval("\$post[avatar] = \"".gettemplate("postbit_avatar")."\";");
} else {
eval("\$post[avatar] = \"".gettemplate("postbit_avataroffline")."\";");
}
}
Using this your members will have a "no avatar selected" avatar if they haven't used a custom avatar and it will still function like the others (Online/Offline low opacity status).
I'll probably update the hack later but I don't have time right now. :P
Here's the noavatar.gif file which should be placed in images/avatars/