In admin/functions.php find:
global $PHP_SELF, $DB_site, $replacementsetid, $gzipoutput, $gziplevel, $newpmmsg;
Replace with:
global $PHP_SELF, $DB_site, $replacementsetid, $gzipoutput, $gziplevel, $newpmmsg, $bbuserinfo;
Find:
unset($replacement);
Add before:
if ($bbuserinfo['usergroupid'] == X) {
$vars['{FINDME}'] = '{REPLACEWITH}';
}
Untest but should work, replace X with the usergroupid, {FINDME} with the text your searching for and {REPLACEWITH} with the replacement.
I assume this is what you want to do, but maybe you wanted to change all <img> tags to show the same image, in which case, ignore the second edit above and do this instead:
Find:
return $newtext;
Add before:
if ($bbuserinfo['usergroupid'] == X) {
$newtext = preg_replace('#<img (.*)/?>#siU', '<img src="imagename" alt="" />', $newtext);
}
Again untested but should work, replace imagename with the filename/path to the image.