Keep in mind, if the user can see the image, they're able to save it if they're savvy enough. So all of this is just security through obscurity and will never completely solve your client's issue. It would be smarter (but more work) to watermark the images, put up a legal notice, and then have the client police any illegal uses.

But of course, if the client asked you to do this, they probably won't go to all of that trouble.

The next problem is the 'timeline' of how easy it is to use for a user versus how 'secure' (in this case 'obscure') it is.

If you still want to do the CSS background-image trick, but in the editor, you would have to teach the client basic CSS and how to do manipulate CSS in the editor. There is no other good method, as I don't see anyone creating some WYSIWYG editor to create markup to fit that model. (As it is bad from a semantic and theoretical view, and is, once again, just going to slow down users who don't want the image that bad.)