Yeah, it's from Battle for Wesnoth (www.wesnoth.org), all of their core artwork is GPL2 licensed. If I remember right, there is some talk in their forums encouraging other open source games to use their artwork. If you are interested you ought to look around their community for the right person to ask permission.
EDIT: Found a related forum discussion: http://forums.wesnoth.org/viewtopic.php?f=9&t=34892&...