Once you make the change to the usergroup title, you have to update everyone's titles and ranks.
AdminCP > Maintenance > Update Counters > Update User Titles and Ranks
Once you've done that you just need to remove the custom title of whoever's in that usergroup. Alternatively, you can just include the HTML for the image in the custom usertitle.
Or you could set a rank for the usergroup such that everyone is awarded the rank.
@Haltech - I'm not sure what mod you're talking about, but I think that if you make your own plugin, you can fix that problem of yours...
Product: [whatever, vBulletin or something else or make your own, doesn't matter)
Hook Location: fetch_musername
Plugin Title: Put group titles and user titles together
Code:
$user['usertitle'] = $user['displayusertitle'].'<br />'.htmlspecialchars_uni($user['usertitle']);
That should display the group title above the custom user titles. In theory. I haven't tried it. If you point me to the mod that broke I might be a little more helpful.