Originally Posted by schoy
When you buy a car, you're not hiring Porsche to build you a specific design. You're paying for a good.
OP's situation is no different than any patent created by an engineer at, say, Apple. Does the patent belong to the engineer or Apple?
Edit: actually, the OP's situation is a bit different because OP is not an employee (but an independent contractor). Wikipedia is our friend here: http://en.wikipedia.org/wiki/Work_for_hire
Without contract, and we agree that verbal contracts can be enforceable, any photo, image, print, raw file, thumbnail, etc... is copyrighted by the artist. My guess is that customer nor OP made verbal or written notice specifically about works for hire. Without that, copyright goes back to the artist. Keep in mind that the OP isn't planning on any legal action. He's just looking to see what stance he should take on the situation and going forward.
Customer in OP's case is viewing the photo as a good he purchased from OP. Not much different than a Porsche. Especially when you commission a car to be built from them. With the thousands of different build options to choose from, you could essentially make it a one off item built specifically for you. Not a perfect example but you see where i was coming from.