You own the copyright to your photographs, which means no one else is allowed to reproduce them without your permission. Licensing is simply the granting of that permission, which can be as a gift or in exchange for money.
Putting a license in writing is highly recommended, so that you have a record of what you are to receive in exchange for the license, and the limits of the license. You may wish to consult with an attorney about an appropriate license for your photographs, or you may wish to proceed on your own. A license does not have to be written in legalese. Plain language is fine, as long as you cover all the important details. Here are a few issues that you may wish to address in your license:
- The payment amount, how you will be paid, and whether you must be paid before the image can be published;
- The specific use you are giving permission for, with a statement that all other rights are reserved;
- Whether attribution is required, and if so, the exact wording of the credit line;
- Whether you are licensing for only electronic publication, only print publication, or both;
- Whether you are granting one-time publication rights, exclusive or non-exclusive rights, first rights or serial rights (when in doubt, one-time rights are more favorable to the photographer);
- The context you are giving permission for the photograph to be published in (for instance, to accompany a specific article);
- Whether you are licensing the photo for commercial use to sell products, or only editorial use to accompany an article; and
- The duration of the license, if it is for a specific time period.
You may view a sample photo license on Docracy.
If you wish to publish your photographs under a general license that allows people to use them as long as they credit you, or as long as they do not use the photographs commercially, or with other general restrictions, consider Creative Commons.