I found this post: How do I delete all generated images from my server except those currently used in posts which describes how to delete all image sizes but the original uploaded image file.
I need to do this on a post by post basis for a custom post type named "gallery". None of the images are in the post content, they are simply attached ot the post. There is no content entered for my gallery custom post type.
How would I go about altering the code in the above post to fit my needs? A simple push in the right direction would be greatly appreciated.