If a file is listed under the Creative Commons Attribution-NonCommercial (BY-NC) License, you may redistribute, remix, tweak, and build upon this work non-commercially, as long as you credit this artist by linking back but you do not have to license your new creations on the same terms.
Creative Commons defines non-commercial as "not primarily intended for or directed towards commercial advantage or monetary compensation."
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.
You must also indicate if any changes were made.
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