[gb2world]

You can use webmaster mode to find the template where the image-box div is set. In this case, when you hover over the template names in webmaster mode, I suspect it will point you to the modules/Detailed_Product_Images/popup_image.tpl
If so - delete or comment out the height setting. You do have to execute care when doing this. If all your detailed images are the same height or less than the product thumb, you should be okay. If any are taller - you need to modify the css to allow for the height when you use the mouse-over of the detailed image thumbs. That is why QT coded this height into the template.