Apply a non-visible unique code on every image render on a website(client side). The code cannot be removed/distorted unless the full image is distorted.
This will help track down who downloaded the image by infringing copyrights or who has bought the HQ image and then leaked it for free.
How? by leveraging pixels colors/patterns and altering them in an un-noticeable way. It can be done in PS by changing image params so that only in specific changes the text/symbols come up; which could be similar to a barcode/qr-code.