Strictly speaking, all photographic work is covered by copyright, and may only be used for the purpose/s that it was created for, and for no other purpose without express permission by the photographer.

However, some photographers choose to waive copyright, which means that they give the client the right to use it for other purposes. It should clearly state this in the contract you sign with a photographer if this is the case. Otherwise the default law is that the photographer retains copyright over their work. Inherent copyright is still retained even if the photographer supplies digital files of the work, however, the contract should include a usage clause which details the allowed usage.

When you ask about entering in a competition, I am assuming that you are talking about something like a 'cutest baby' photo competition, not a photography competition? It is unlikely that a photographer would allow entry, as the competition organisers specify uses which countermands the photographer's inherent copyright. It can not be guaranteed that the organisers would respect a request to give credit to the photographer, so it is not in their commercial interest to allow it. Many competitions such as this have an entry clause requiring that only work by the entrant may be submitted anyway.