icons.png includes icons from the CKEditor project which are triple
licensed with GPL, LGPL, and
MPL:
http://ckeditor.com/about/license
icons.png also includes icons
from the Fugue set (http://p.yusukekamiyamane.com/) which are released
under the Creative Commons Attribution 3.0
license:
http://creativecommons.org/licenses/by/3.0/