How to improve product thumbnails on product listing page where the products are legal documents?

I am trying to understand what improvements I can make to a image icons on a product listing page. The challenge is that the products being sold are assessment documents and templates. Currently all the products feature the same illustration of a stack of paper with the front page containing lines, a simple stamp and a squiggle to resemble a signature.

It is not feasible to place a preview of the document, I don't know what to put in its place to improve. Does anyone have any insight into how to make improvements to a product image on a listing page where the product isn't as glamourous as clothing for example. What can I research and how would I test to see if the changes made has made an improvement?