[EvilLS1]

What do you mean by "depending on whether a user belongs to one of these categories".. By catagories do you mean usergroups? If so, just create the new usergroup, then in functions.php

Find:

eval("\$post[avatar] = \"".gettemplate("postbit_avatar")."\";");
}

Below that add:

if ($post[usergroupid] ==X) {
$image = "<img src=\"http://www.yoursite.com/forums/images/specialimage.gif\">";
} else {
$image="";
}

Just replace the red X with the usergroup ID # for the one you want to have the image. The in the postbit next to the avatar add $image .

You could also do it based on custom profile fields if thats what you mean. But you'd need to add the drop down profile field selection hack first.