Module is now updated and works for both 4.2.x and 4.1.x!
Please see
http://simplexcart.com/modifications/highslide
If anyone has problems or questions, do let me know and I'll gladly help you get it working on your site
Also, original posts for 4.2.x and 4.1.x modifications will still be kept in the thread and on the site, however it should only be used as a reference.
Will be working on a newer version with even
moar features. Not going to go into too much depth but it will provide an all-in-one solution for modal windows. Per page, category control as well...