There’s no reliable way to fully prevent copying once an image is displayed on a client’s screen. If they can see it, the image data is on their device somehow, and screenshots are controlled by the operating system, not your gallery alone.

You can make copying a little harder, such as disabling right-click, but that’s only a minor obstacle and is easy to bypass. It won’t stop screenshots or other methods.

The most practical alternatives are:

• show low-resolution, lower-quality proofs
• use visible watermarks
• display only small thumbnails, or a thumbnail plus a small full-quality detail crop
• rethink your sales model so you’re not relying entirely on print sales from proofs
• consider offering digital files at a price that covers your time if that fits your market

In short: you can discourage casual copying, but you can’t truly stop it online. The best protection is limiting proof quality and aligning your business model with how clients actually buy.