Copyright law is notoriously a minefield, especially if matters of ownership and future usage weren't clearly defined in a contract prior to the creative process taking place. This is an issue that's been further complicated by the advent of new technologies and media, which may not even have been conceived of at the time a deal was originally negotiated and defined. These days, you'll commonly come across scary Big Brother-type clauses in rights-ownership contracts that require the handover by the creative gun-for-hire of: "all rights, in perpetuity, throughout the universe, in all languages, and in all media now known or hereafter devised."
If you had already taken the photographs independently and were subsequently approached by a third party wishing to use them, then that's a different kettle of fish, and it's between you and the casino to negotiate the extent of any rights, which will reside with you, the "author," until you sign any of them away. The scenario you outlined seems to be of somewhat narrower and more clear-cut parameters, however: If you were, as you indicate, specifically commissioned by a casino entity to take publicity photographs on their behalf, then you can most likely kiss goodbye to your photographs as far as any future commercial exploitation by you is concerned.
Here's the deal: Although in most instances, as the creator of a work you're recognized as its author and, hence, the copyright holder, if the item in question was a "work made for hire," (officially defined as "prepared by an employee within the scope of his or her employment or a work specially ordered or commissioned in certain specified circumstances"), then "the employer, or commissioning author, is considered to be the author" (U.S. Copyright Office, FAQ), unless you separately negotiated a non-standard clause waiving your contractor's standard rights.
For future reference, if you didn't negotiate such a deal, you should certainly consider trying to for any future assignments, especially if you're taking a bunch of photographs of which only a handful are going to be used by the contractor. (As an aside, if these were publicity photos taken at an event that featured celebrities, for example, then that opens up a whole other can of worms in terms of what usage the personalities photographed may have agreed to, or otherwise.)
Bottom line, check your contract, which should be clear on this point -- most likely all the images you took belong to your paymaster, although you're probably free to include them in your portfolio as a marketing tool, for example, just not free to re-sell them to a different third-party entity.
It's always worthwhile trying to negotiate non-exclusive rights deals, of course, if you think the images have a shelf-life and scope for future generic usage, although that will almost certainly reduce your initial fee, if someone hired you. Attempting to go that route is a judgement call on your part, but if you've already signed a contract in this instance, check the terms -- it almost certainly defines the paying party's rights as exclusive, whether or not your pictures ever see the light of day.
P.S. We're intrigued about what these mysterious photographs are of, so please let us know -- contract permitting :-)
