I once thought this would be important as my collection of images grew large. I spent quite a bit of time looking for a good solution, even participating in a beta test of some database software tailored toward photographers. But in the end it turns out that, unless you are a professional photographer with magazines and ad companies coming to you with specific requests, this is something that is never done. You may go looking for a specific tree-in-a-sunset photo that you took 5 years ago. But the fact that it was during a Serengeti safari will be adequate to find it using my scheme.