Change popup large image link

Hi guys. Sorry if this question has been posted, I tried to search for it but Im not sure what keywords describe the problem.

I want to change the large image link that pops up in a new window when you click on a product thumbnail.
Normally the thumbnail is a scaled down version of the larger popup image, I want the larger popup to be a completely different image file (my product images look terrible when scaled)

Thanks in advance.

Try reading through the thread on the calicocat that demonstrates the usage of the small, medium and large images plus the additional images and additional large images ...
http://www.zen-cart.com/forum/showthread.php?t=41010&highlight=%2Acalicocat%2A

Thanks Ajeh, I have read that thread already, and while helpful for listing more then one image, my problem is I want my thumbnail image to link to a completely different image all together.
I am presuming I will need to alter the JS, but thats a little out of my league without help.

Cheers