Custom Fields in Photos?

I'd like to know if it is currently possible to add a custom field to a photo that the uploader can fill in. I'd like to have a "Photo Credit" field so that is a person uploads another persons photo, they can give credit to that person. I'm building a first responder website and in our hobby, it is important for people to be able to give credit to photos that were taken many years ago. If this isn't possible, I suppose I can require them to put it in the description, but I'd like to have a special field for it that I can make required if possible.