See each file for license.

large-alert.png, large-info.png, large-stop.png,
alert.png, info.png, error.png:
  GPL.  See
  <http://www.lullabot.com/articles/free_gpl_icons_lullacons_pack_1>
  for the full package.  See
  <http://www.lullabot.com/articles/creating_awesome_new_icons>
  for the source distribution.

ok.png:
  LGPL.  See
  <http://www.iconfinder.net/index.php?iconid=1236&size=32&q=OK>.