Hi Lee,
One thing you can do is to use some CC licenced images just for illustration purposes on your theme; and then, in your final file, leave a placeholder for buyers place their own images (be sure to put in the item description that images are not included). You can even suggest some stock images to the buyers by placing a link in the help file (if they want to buy those images)
At least, that is how we manage the images on Graphic River.
Best regards,