The main version of XBoard has been GTK based for many years now. But all versions of XBoard use the same graphics formats. The primary format is SVG, so the images can be used for any board size. XBoard would also be able to use PNG format, but this can be used only for one particular board size without loss of image quality.
Compatibility of licences is no issue, as the graphics files are not part of the program binary, but loaded as external data. The XBoard licence (GPL) does not put any restrictions as to what data the program would be allowed to process. So the only thing that matters is whether the graphics files can be freely used and redistributed.
Op vr., feb. 12, 2021 om 07:52, "mayuresh@kathe.in" <mayuresh@kathe.in> schreef:
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)?