Protecting images?

[Froadin]

We have many html and pdf pages that we need to protect on our site.
It is one of the reasons we have a paying membership. And we want to make sure that all our time into these images (diagrams, schematics, etc.) is not wasted by someone just copying them off.

How to best do this?
Right now we are putting a clear gif image over the top of each one. But this is limited.
I was thinking that we could load all the images into the database, then we can control how they are seen on a user level basis.

We accept the fact that paying members will most likely download the images to store on their computer. But we would like to keep that from happening if possible (or offer it as an extra service).

Are there any plugins that can manage this?
I took a quick look at the files download manager, but I am not sure this will solve my issue.

When we used Postnuke before, we just used the PostWrap plugin, and it did a real good job of hiding where the directory was. Plus a few other tricks, and we were ok with that.

It just seems that VB is powerful enough to offer a GOOD way to protect images/files, and have good management over them. Meaning, we have show them as images within an HTML page, but still have complete control over who can see them and download them.