How do I Protect Downloadable Merchandise from Being Pirated?

I sell cartoons on a WordPress 4.2.1 site with a WooCommerce Storefront theme. You can see one of the product pages at https://mchumor.com/cartoon/6990-computer-cartoon/. After customers pay for a cartoon they are sent the filepath-link to an un-watermarked jpg that they can drag and drop. All 4,000 plus jpgs are in one folder. I worry about the day Google discovers the folder and thus the world may know how to access my cartoons. From what I’ve been able to glean from these forums, one solution is to password protect the file with the jpgs by putting code in an htaccess file and putting that file into the image file (how many times can you use the word file in a sentence before it becomes a run on sentence? … but I digress). Problem is, I haven’t been able to figure out exactly what code I should put in the htaccess file and how that affects customers ability to download a cartoons. Thank you. Humble cartoonist