It's not hard coded in any of the files. Go to Administration > System Admin > Groups. From here, edit the Webmaster group by clicking the edit link. On that page, in the text box, replace Webmaster with the title that you wish to give your Webmaster group.
I'm not sure if the code is actually in there for this type of thing already, but I know that in XOOPS v1, these buttons were used in the forums. The person who made the new XOOPS v2 icons probably just included the images in there because they existed before, but the code may not be in there.
Before you replied, I tried to go in as a webmaster and change the theme to the default XOOPS one and then switch it back; that seemed to rectify all the problems.
onokazu wrote: This cannot be changed, unfortunately. The users are always registered to a group with an ID defined for XOOPS_GROUP_USERS in mainfile.php of your site.
All you'd have to do is change the XOOPS_GROUP_USERS value to the new default group number, theoretically. I'm not sure if this would mess anything up in the XOOPS core, but I think that should do the trick.
I fixed this a while back in my personal copy of XOOPS; all you need to do is open up your template for newbb_thread.html and change where it says <{if $xoops_is_admin == true}> and make it say <{if $viewer_is_admin == true}>.