If a file is listed under the Creative Commons Attribution-NonCommercial-ShareAlike(BY-NC-SA) License, you may redistribute, remix, tweak, and build upon this work non-commercially, as long as you credit the 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.
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