For mockup images, you can use an unmistakably prominent watermark. Superimpose the word "MOCK-UP" in a gigantic font that takes up nearly entirely the image.

For mockup images, you can use an unmistakably prominent watermark. Superimpose the word "MOCK-UP" in a gigantic font that takes up nearly entirely the image. Make it translucent and red or make it black outline. And maybe tilt it diagonally to catch attention and to make it easier to visually separate it from the rest of the image. To streamline the process, you can just keep an image like this around to import as a topmost layer into other images.