Free art assets
Free art assets are the graphics, sound, music, fonts and 3D models in a game that are distributed under a licence granting the same freedoms as free software. A game whose code is free but whose art is not cannot be redistributed in full, so asset licensing decides whether a project can be packaged by a distribution at all.
Licences in practice[edit]
Art is commonly released under Creative Commons terms, most often CC0 or CC BY-SA, or under a free software licence such as the GNU General Public License applied to the asset files. CC BY-NC and CC BY-ND are not considered free for this purpose, because the non-commercial and no-derivatives clauses fail the Debian Free Software Guidelines: a distribution cannot restrict use to non-commercial purposes, and it must be able to modify the work.
Why it is difficult[edit]
Illustrators and musicians work in formats and with habits formed outside free-software practice, and source files are often lost once a game ships, which makes later modification impossible in fact even when it is permitted in law. Projects that keep layered sources, and that document the provenance and licence of each submitted file, are the ones that survive packaging and years of later porting work.