Interesting question. The studio that does the photography for my kids' school pictures will sell a CD to you with the school picture on it. Along with the CD (which in the digital times we live in is like a negative) they include a release granting you rights to reproduce the picture. So I would lean towards the copyright remaining with the photographer/studio, while the rights to reproduce the image may or may not be licensed to the purchaser depending on the terms of sale.
Personally (and I am not a lawyer so this is just my opinion), if I were you, and included the negative in the sale, I would spell out whether or not the rights to reproduce the image are transferred to the purchaser. This makes it clear to both parties exactly what rights, (if any) are being transferred to the purchaser of said image.