Due to original design it was developed with opening the the image of that thumbnail and unfortunately there is no option to a set a different image to be opened and the feature you are seeking could be achieved only via custom work. Unfortunately we do not provide our customers with any custom work due to our policy,therefore we do not provide any instructions or files list for implementing customization you are seeking. We only do Support for our products if there are some issues in work, or some elements cause issues, etc.
In case you need to make some customizations you could try to figure this out on your own, which needs some skills in coding and programming.
In case if you believe our theme misses enough custom options to fully meet your needs you could involve/ask for help a third part developer to customize the theme the way you personally like.
Thanks for understanding and sorry for any inconveniences!