First off, there's the mechanics of how to do it. What tools does he need? You know that there's a hammer and nails in the back of the supply closet. He doesn't, and it's fair of him to assume that you wouldn't have asked him to do something he didn't have the tools for. He looks around his desk, and he's got a stapler and a tape dispenser.
There is also another way this can go wrong, particularly with a certain breed of eager young programmer. You find that he's gone down this path when your boss comes by the next week to ask about this purchase order for a nail gun. So you talk to your guy and discover that he's spent the last week Googling, reading reference works, and posting to news groups. He's learned that you hang pictures on a nail driven into the wall, and that the right tool for driving nails into walls is a high-end, pneumatic nail gun. If you're lucky, you can point out that there's a difference between picture-hanging nails and structural nails, and that a small, lightweight hammer like you have in the supply closet is really the right tool for the job. If you're not lucky, you have a fight on your hands...
So now we get into higher-level design issues. Where should the picture go? At what height should it be hung? He has no way of judging any of this, and again, it's not as obvious as you think.