You actually create a real .jpg image with some text or pattern embedded on top of the photo.
Anything you display on the internet can be copied. You can only control the quality of the image.
You give users a watermarked, reduced resolution, or crappy quality.
If they copy it, that's what they get.
You may want to create a graphic image for a user that enters text on a form.
Here's sort of a 'sappy' example:
The idea is to show the rendered image is really an image, not HTML.