I took a quick look at your site, and here's what I see:
It appears you're using TB (ThickBox) as a plugin, and it also appears that you have multiple instances of TB loaded.
You have this one:
<script type='text/javascript' src='http://consequenceofsound.net/wp-content/plugins/wp-shopping-cart/js/thickbox.js?ver=Instinct_e-commerce'></script>
And then you have this one:
<script type='text/javascript' src='http://consequenceofsound.net/wp-includes/js/thickbox/thickbox.js?ver=3.1'></script>
I suspect that's not a good idea. I couldn't quickly find an image that clearly should have been loading with TB. I tried the images in the sidebar (Springsteen, etc) and they didn't load with TB, so it's not clear that TB is working. The 2 instances of it might be why.
In my configuration, I'm loading TB not from a plugin, which simplifies things in a way. For you to sort this out, I'd try to eliminate 1 of the 2 TB instances, and then get in touch with the plugin author.
To answer your last question, your link with festival-outlook in it is from a page with that name. You'll know it's working when it doesn't generate a bad link. The plugin isn't following the standard TB behavior; maybe there is another setting for the plugin? If you were using only the javascript TB (rather than a plugin), the fix you made would be done.
Btw, here's a link to the forum of the original developer of TB:
http://codylindley.com/thickboxforum/
Since you aren't using TB by itself, they may just refer you back to the plugin author, but the folks there know a lot more about TB than I do.
Good luck.