You can choose one of the following Creative Commons licenses to apply to your icon during the upload process:
Creative Commons Attribution (CC BY) allows you to let others download, use, copy, share, modify, and build upon your icon, as long as they either 1) credit you as the creator, or 2) pay a small fee to use the icon without giving you credit.
CC0 - No Rights Reserved is a public domain dedication tool which allows the creator or owner of an icon to declare the work free of copyright. Placing an icon into the public domain is NOT revocable, so make sure to read about CC0 first.
Public Domain Mark is a tool that lets you identify an icon that is already in the Public Domain and is therefore free of known copyright restrictions. An example of this would be an original work of the US Government (for example the AIGA transportation suite), or symbols that have already been released into the public domain (such as the Iconathon suite).