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

an associate of mine is considering freeing the graphics assets he'd go=
t developed for one of his commercial chess product.
if those assets (board + pieces) have to be usable (maybe as default) w=
ith xboard, which license is the most compatible for releasing them? Al=
so, which image format should he release them as to allow quick and eas=
y integration with the existing xboard (as opposed to the upcoming gtk+=
 based)?