When you read competition rules it should be clear from reading them whether or not rights to use your images commercially are required. If commercial usage of your work is required the rules should make clear what commercial usage rights you are required to grant.
Note that free use of your works should be solely to promote the contest and no other purpose.
There is nothing wrong with organisers making commercial use of works you submit to a competition as long as the organiser negotiates the terms of a rights managed license license with you.
Without a license from you other people or organisations are not legally permitted to use your work. In a rights managed license you can specify limits on usage including -
the agreed use to which your work is to be restricted (e.g. a book cover),
the date on which your work must cease to be used,
in which countries your work can be used,
how much you want paid (the usage fee) for the requested rights.
These are important and valuable rights and you do not have to be a professional artist to enjoy them.
To sum up, competition rules should make clear to you -
If you would like to know more about copyright and related rights please read our Guide to Rights and Licensing.