I just came looking for how to do the same thing. I just purchased the program and will be selling video templates. The videos as demos are in the description, so images are unwanted. If it didn't display "Image Coming Soon" everywhere it would be fine.
if you are still looking for a solution you could always modify the default images associated to an invisible or transparent .png its quick and easy... the images are located in yourwebsite.com/media/com_eshop/products/resized/no-image-500x500.png and a few other sizes.