thumbnail on the review page? Actually if your talking of the thumb display underneath that is not advisable unless you want to break the functionality of the product
On showproduct you have the ability to show 3 images that were uploaded for each product. Those thumbs are clickable in a javascript which alternates the views to that main image you see. Try uploading a product with 3 images and test what I mean.
At any rate you can ask specific questions on how to hack your install at
http://www.photopostdev.com since that sight deals with hacking the code. This is a support site for the normal functionality of the product.