From the front page of the web site :If you find any of the images to your liking, please feel free to download and use them for whatever purpose with the proviso that their use conform with the copyright rules governing Attribution-ShareALike 3.0 Unported as described by
Creative Commons in their Public License.
to share – to copy, distribute and transmit the work
to remix – to adapt the work
Under the following conditions:
attribution – You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.
share alike – If you remix, transform, or build upon the material, you must distribute your contributions under the
same or compatible license as the original.