Nice topic. As mentioned in this article, anything greater than CC0 requires citing the source, however it also requires a link to CC license being used.
Attribution Example:
*"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. "
There is an attribution link with all of the items above on the download page from the CC website when selecting the image. --3D