Attributions:

svg/tiles/lab/biohazard.svg
 * public domain
 * http://en.wikipedia.org/wiki/File:Biohazard.svg

svg/tiles/lab/exit.svg
 * "The symbols presented in these guidelines can be freely used by anyone and
   can be accessed at the Web site (http//www.ecomo.or.jp) of the Eco-Mo
   Foundation. However, registering these symbols as your trademark or design
   can be a copyright infringement."
 * http://commons.wikimedia.org/wiki/File:PublicInformationSymbol_EmergencyExit.svg

All other images are original and licensed under the same terms as the rest of
the game.