You need to have an explicit work-for-hire agreement in place -- and if you can't find that, then the copyright holder is unknown. So there's another source of orphaned works -- one where the work-for-hire agreement is lost.

Most low-end photographers these days charge a stupid sum for such an agreement, though, as they want to control the reproduction rights of their works. Try it at a mall photographer's booth. (I suspect wedding photographers are different.)

But, the take-home point here is that just because you pay an artist to come to your event and product art based on or inspired by your event, and give you copies of said art, doesn't mean that you own the copyright on that art -- the artist has to explicitly transfer that copyright to you.