If it's on the web, it's there to be taken, not much you can do to stop this. If you are using images that are your own work, like photography, then a good option would be to watermark them using proper software. This merges the mark into the pixels of the image so it cannot be removed (unlike graphic editing software which only places a layer on top). That may not stop the thief, but it will mark it as your property for proof of copyright violation.

safeTsurfa
Stay alert! Stay safe! Stay alive!