This is all assuming that you can take a picture of a ghost with a normal camera, which I think you can't. Anyways, if you see no ghosts then you cannot take pictures. Which puts you back to square one. But if you see a ghost and take its picture yet nothing appears on film, then you seen a ghost with your own eyes but have no proof. So this whole thing kinda seems sadly pointless.

There are probably many ways you can achieve this but I tend to respect the dead. I actually have a few ideas in which you could try but I am not going to say them, since I do believe in ghost. I say do not mess with them since you will be asking for more than you can handle. Then again, maybe I am crazy. :/