I think this question can have better answers in another site, like Security StackExchange. And there you'll get answers that, in the end, will tell you the following: it's impossible. If you really don't want anyone to copy your pictures, you can't let that person touch it.
Then the long answer will be the following:
Security is a matter of trade-off: you trade security, user experience (easy-to-use vs hard to use) and willing to break your protection.
If you're really afraid on what the person can do with the picture, you shouldn't let him take the picture. Because he can take a photo with another camera (and you'll never know if he has used his own equipment, cell phone, so on). And he also can copy those memory cards before handing to you, so you won't event know he has a copy. Everything you try to protect, neglecting this first point, can't be effective, since this would be the first way to circumvent it.
If it's ok for a 2nd person to take the photo, but you don't want that a 3rd one can copy it: you can't let this person have access to it. If he is able to open it on his computer, he can: burn it on a dvd, copy it on a flash drive, upload it to some site, send himself using email, send to anyone using some chat program, hide it inside another photo he has permission to copy (layers on photoshop?), print it, take a picture of his monitor using a cell phone, film the monitor, play it using headphones and then record it using some microphone or the old telephone, copy it like a text, write it down using pencil and paper.
Is that all easy to do? No. Some methods might require internet access, some might require installation of some software, some might take a long time.
And if the person doesn't have access to the picture, but anyhow works for you using some computer in your network? He can install some kind of virus, he can sniff the network, he can attack the NAS... Well, if some of your computer is accessible from internet, the person doesn't even need to work for you or be inside your company.
So, the trade-off is: you can't be sure that the photos are really protected. How hard you want to make it for someone who is trying to copy it? In the case you described, you trust the person (not that trusting really means too much), you just want to make it harder, right?
Give him just a keyboard, a mouse and a monitor. Be sure to make the computer physically unreachable, then you'll need to worry less about cd burner, flash drive access, etc. Then take all the internet access away, so that he can't communicate with the world. Configure your computer so that he can't install any software. Make sure he can't have any electronic device with him, so that he can't take a picture of the monitor. Not only electronics, really: old film cameras are also a problem.
And then be prepared to have some employees leaving your company. When security policies are that strict, including not allowing personal devices, people tend to get upset, bored, and search for happier places.
