Link a field in content types to the user profile
First, you should make sure that the default author relationship isn't sufficient to satisfy your needs. Every content type has a creating user; this relationship can usually be used to create any necessary links between the content and the user in Views, Panels, or other modules.

However, sometimes you need to have multiple users associated with content. Your "photographed by:" example sounds like a good example of this use case. In that instance, I would use the User Relationships module to add a User Relationship field to the content type.