It is not possible.
For example, you have a picture posted on your website.
If your website can be viewed publicly (viewable by ANYONE), the user who's viewing your website can simply right-click and save your images.
BUT WAIT, you can have a setting where people can't right click on your website!
But then again, they can just hit the 'print screen' button on their keyboard and they can have your picture saved on their computer.
One suggestion is: when posting a picture on a website that you dont want others to copy, add a watermark across the image, make it smaller so they wont get the big, magnificent picture.
So when they want the picture that you previewed, you can just send them the big one without watermark.
I hope it helps.