This is an accepted solution.
Do not know you've found the app to solve this problem yet? You can try this one https://apps.shopify.com/super-watermarks
I used it for my store to batch add my logo or sticker to product images
Hope you can solve your problem.
You're not going to be able to prevent people from downloading your images if you add a watermark in your code. You will need to put a watermark on them in a graphics editing program. You can put your logo over the images with HTML and CSS, but when the user downloads the image it's not going to download your watermark as well unless it's actually a part of the image. In which case, they could still use a graphics editing program to just remove it (assuming your pictures are on white anyway, and assuming the watermark doesn't actually appear over the product).