If a file is listed under the Creative Commons Attribution-ShareAlike (BY-SA) License, you may redistribute, remix, tweak, and build upon this work even for commercial reasons, as long as you credit this artist by linking back and license your new creations under the same terms.

Under this license you must give appropriate credit. This means: if supplied, you must provide the name of the creator and attribution parties, a copyright notice, a license notice, a disclaimer notice, and a link to the material. 

If you remix, transform, or build upon the material, you must distribute your contributions under the same license as the original.

Here's a perfect generic example of attribution (modifications are necessary for each license):

"Creative Commons 10th Birthday Celebration San Francisco" by tvol is licensed under CC BY / Desaturated from original