You should never modify the template that comes with a module. Override it in your themes or mysite directory. Leave the module code untouched so that you can cleanly update the code as new versions come out. Especially if you're on SVN -- you'll run into conflicts that way.
None of the templates that come with modules are intended to be the final implementation. You're expected to override them with your own styles and markup. Just recreate ImageGalleryPage.ss and ImageGalleryPage_album.ss in your theme folder and you should be fine.