Taken from a reply I posted on a facebook group discussion
In most (but not all) countries, image rights automatically rest with the photographer, exclusively, unless there is a specific contractual exception to this (including employment contracts, where the employer typically takes the rights).
Usually clients get a license from the photographer for specific usage for specific images for specific purposes. If you are paying the photographer, you are the client but this does not give you any rights to the photographs unless (and under the very unlikely eventuality) the photographer hands all rights over to you or unless you get a license (not much point being the client if you don’t though).
If the photographer is paying you, well, all bets are off. See what you can negotiate. If you do not like the deal, walk away. A model release is for the benefit of the photographer (or rather the subsequent users/publishers of the photos who will look to the photographer to provide such a release), not the model. It removes any rights you have to challenge use of the photos (including advertising use that you may not like) unless explicitly ruled out in the release – and note that any such challenge is against the user(s)/publisher(s) not usually the photographer unless s/he is the user/publisher or has misrepresented you in some way.
You need a license from the photographer to be able to use the photos yourself (and that will be under the terms the photographer offers). Unless such a license is given in perpetuity, it can be revoked at any time. The exception regarding a license comes where there is an implied contract such as under a Trade for Print/Photo/CD arrangement where there is clear intent to provide mutual benefit. This can be tricky to prove though, even if the photographer hands you several very nice prints, so it is better to always get a license to banish all possible doubt.
Nude means many things (and there are lots of terms used to agree exactly what can and cannot be done e.g. OL for “open leg”). You should check out some of the modelling sites for a variety of definitions and also advice on your safety. Google “model mayhem” and “purestorm” as a start.
Think very carefully before you do nude photography given what I have said above. Your ability to control what happens to the images in the future is very limited.