[a3_nm]

Just to be precise, only the game engine is open source. "Images are licensed under a Creative Commons License and are provided separately," and this CC license is BY-NC-SA which is neither FSF- nor OSI-compliant.

[zitterbewegung]

This looks like a good thing for the liberated pixel cup to be used to replace those assets. http://lpc.opengameart.org/