I made this a while back for my own gallery. So far it prevents copying as far as I know without using brute force and digging through cache or screenshoting the browser. I thought I might share and perhaps someone may help improve it as well.
Although it does exactly what I intended it to do, I cannot call it an actual plugin or add-on. The install is a bit lousy I admit.

Sadly it modifies to coppermine files. To use it, you must include either the code or the file containing the code, into your theme file. I haven't really had a chance to look into how to make it a plugin, but more or less I'd like to not have to be able to modify the following files, but just overwrite the script/functions:
profile.php - for the thumbnails
/include/slideshow.inc.php - the whole script
Nonetheless it works great - been using it for a month myself. Hopefully it helps a few others out there as well.
Coppermine Anti-Copy ModHere it is working on my own site:
GalleryCurrently I have the following set up on the mod:
- Basic Protection: Overlay a blank image over all pictures (prevent right-click copying)
- Hiding Links: Pass all picture addresses through an image-generating file:
--* Requires random code from server to generate an image.
----* Random code generated from timestamp + offset from any resource of your choosing
--* Optional caption on generated images.
--* Optional resizing on the fly.
--* Image-generator and its features usable independently from mod and gallery (see CUSTOMIZATION)
- For those with Apache: Deny access to files
--* Deny direct access to all image files (specify types in .htaccess file)
--* Forward those who find the images to a page of your choice.
--* Allow you to rename the image-generating file to any name and extension.
- Copying Permissions: Specify specific users and groups to allow copying
--* Enter the user id # or group name into the Gallery Description or Album Password Hint
--* Seperate with commas, no spaces (unless part of group name)
--* Place a negative in front to deny. ex -1, -banned
--* Denying has priority over allowing.
--* Optional removal of caption for those allowed to copy, see CUSTOMIZATION
- Extra features I added to my own gallery and am too lazy to undo:
--* Categories with no visible albums (to the user) and no thumbnail/description become hidden from view.
--* Videos default to 640x480 dimensions
--* Non-image files can now use custom intermediate images if uploaded (default normal_ prefix)