Since all material is viewed locally on one’s machine, if the user can ‘see’ the image, then it means there is already a copy that has been downloaded to the user’s machine. (the client’s machine downloads all the material to view the page to the local machine, then shows that material to the user, so they are never directly ‘seeing’ the information on your server)
That said, that are some schemes to try to obfuscate the location of the image, but these are usually more trouble then they are worth. Besides the user needing to download the image to view it anyways, self-defeating.
The more universal method is watermarking the images, or modifying the image some other way like lowering the resolution to some point where it is no good but for previews.