(FYI, it's over a decade ago, so my memory is a bit foggy, but I believe when using miva cart with my book business, I didn't use their image handling system. Instead I edited the global head, pasted in a javascript for popups, wrote a html link into the product page around the image tolken, set up an extra field with the relative url and used a tolken to place that extra field into the link, and when you clicked on the image, a popup of a larger image came up for that book)