png2cv is a tool that creates ColecoVision characters for bitmap mode from .png images. png2cv is covered by the GPL.

Discussion: http://www.atariage.com/forums/topic/152306-png2cv-0-13-reducing-memory-usage-of-pattern-and-color-tables-a-bit/

http://colecovision.eu/ColecoVision/development/png2cv.shtml