Using additional template pages

I would have thought that this would have been covered before, but I searched for most strings that I could imagine that relate to the topic, and couldn’t find anything.

I created a plugin that takes a Picasa exported gallery and displays it on a post, just by using the add_filter api hook. The gallery is constructed of thumbnails on the main page, and then each thumbnail links to the full size image.

I’d like to make this a little bit more sophisticated now and have the thumbnail link to another page that maintained the theme of the site, displayed the image in the correct location, based on the theme itself and also some previous and next buttons to link to the corresponding images.

I’d like to do it “within” WordPress. That is, have a custom template page, say image.php in the template directory, and just link to http://www.site.com/?image=file (or a corresponding URL that makes use of the rewrite rules) and have WordPress automagically know to use the image.php template file.

I’ve had a quick glance through the source, but I thought I’d find out if it’s possible without hacking up WordPress. I’d like to do it without hacking, but if I had to, then I guess I can, but that makes it less useful for the general public, and I know of a few people that’d like to use it for their own site.

Any ideas?