There are several ways to do watermarking.

If you want the watermark to be a real security protection (so that if someone was to download the image locally, the watermark would still appear on the image), you have to develop some server-side script that merge the original image with the watermark and shows a "modified" image. This can be done with software like Imagemagick or Photoshop.

You can also add "soft" watermarks. It is not a real security protection, because users can save the the image locally and you will see it without the watermark. But if you only need the "warning" it is enough. You can do this with css or javascript.