xboard : classical chess : graphics assets : donation : preferred/desirable license?

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an associate of mine is considering freeing the graphics assets he'd got 
developed for one of his commercial chess product.
if those assets (board + pieces) have to be usable (maybe as default) with 
xboard, which license is the most compatible for releasing them? Also, which 
image format should he release them as to allow quick and easy integration with 
the existing xboard (as opposed to the upcoming gtk+ based)?