FreeImage is licensed under the GNU General Public License, version 2.0 (GPLv2) or version 3.0 (GPLv3), and the FreeImage Public License (FIPL)
You can choose the license that has the most advantages for you:
Use the liberal FreeImage Public License to use FreeImage commercially or Use the GNU General Public License to use FreeImage into your open source project.
A plain-text version of the FreeImage Public License can be found here. As stated in § 6.1 of this license, Floris van den Berg may publish revised and/or new versions of the License from time to time. Each version will be given a distinguishing version number.