XF 2.1 Custom bbCode built off existing?

[Grahame]

I want to edit the [ GALLERY ] bbCode to allow for the embedding of a thumbnail rather than the full size image with the media data below it. I thought that by editing the gallery.php file in XFMG/BbCode and adding a "thumb" if statement in addition to the "media" and "album" ones I could get it to point at a different rendering template, but changing the GALLERY tag in the post to "thumb" just results in the tag not being parsed at all, so there's obviously another place I need to add the "thumb" option. Can someone point me in the right direction?