I work in IT, so I can tell you how its done.
Short Answer: You use a script that disables right clicking. Paste the script from the link on your site and users can no longer right click an image to save it. This will stop most users.
https://www.hacksparrow.com/disablin...avascript.html
If you want the pop-up effect like then add this other script called Lytebox.
http://www.dond-wp.co.uk/lytebox.html
Long Answer: Any image that can be downloaded by your browser can also be saved locally. Theres multiple ways, ex: intercepting the network request, disabling Javascript, etc. So don't use this for anything you really don't want to be copied or stolen. To prevent theft of images consider using a watermark, so others can at least recognize the source of the stolen image.