Renaming a lot of files can be a real pain.  A post on Greg Duncan's blog reminded me of my favorite, simple called Flexible Renamer.  It hasn't been updated in awhile, and the rest of the site's Japanese, but it works really well, and has functions for images, music, and other files, with support regular expressions.  All operations are real-time previewed before the renaming operation takes place, and several renaming templates are included.  It's definitely worth a look if you ever try to rename files enmasse:

http://hp.vector.co.jp/authors/VA014830/english/FlexRena