This appears to have something to do with FireFox. When I try your link in IE7 it works. Also, in Firefox, if you click on the thumbnail and get 'loading....' then click on the thumbnail anywhere but on the loading icon, the full sized image displays in the browser window.
This could be either a problem in Highslide JS or a conflict with Firefox, Highslide JS and something else that you have loaded on that page.
You might try the newest version of Highslide JS, 3.3.19 just released on June 1. To do this, download the highslide-full.js and highslide-full.packed.js files from vikjavev.no/highslide/download.php
and move them into your /plugins/content/highslide directory. That is where the 3.3.18 version of these files are. The changlog for 3.3.19 does reference a problem with Firefox that was fixed, but the problem description doesn't match your situation exactly.
Please let me know what happens.