What does it even mean to make data files "open source"? Do you mean the specifications of the file formats? The original project resources that went into creating the assets? Or are you confusing Open Source and Free (Libre) with fee (gratis)? Open Source or Free (Libre) means (among other things) that you have access to the source code of the binary, i.e. you can see how the machine works. An asset is not a program, it is not executed, but simply loaded into another program, so it does not make much sense to speak of a source code of an image file.