The problem
A watermark alone doesn't stop the screenshot.
Most online proofing galleries protect a photo by stamping a logo on it and hoping. A client presses two buttons, crops the corner, and your proof is a phone wallpaper, a social post, or the print they never ordered.
Disabling right-click helps with desktop browsers, but it does nothing about the screenshot shortcut, the phone's screen recorder, or a screen-capture app. That's where most proofs actually leak.