I think you should set your images folder and the BMZ_Cache folder at 755 for security reasons. Note it doesn't have to be 777 to be writeable since i have it set to 755 and it works perfectly.
In your zencart admin section go to tools and image handler 2 and clear the image cache.
There is a lightbox addon ( one of several ) which sounds ideal for you as it works with additional images and will make your site look better so it's worth hunting that down and searching these forums for more information.
Lastly i reall think your larger images should be larger than you currently have them set for...Too small in my opinion.
Medium could be slightly larger as i think it's too small but thats your choice.
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