In “Languages of Art”, Nelson Goodman distinguishes between allographic and autographic art. Allographic art has a score in some form of notation (such as music, drama, literature and in some cases dance). Autographic art does not have a score or notation (such as painting). It is immediate in a way that allographic work is not.
Stallman’s Free Software is based on the availability of source code. Source code is a form of score, so computer programs are allographic. “Open Source” is actually “score disclosure”.
This may have some bearing on making visual art copyleft. Paintings are autographic and so have no “source” to open, no code to free, no score to disclose. This does not make Free Art impossible by any means. Paintings may be copied, sampled and derived from. It is important to protect and enable this to continue as it has throughout history.
Although there is no score for a painting, there is the preparatory work. Sketches, maquettes, source material. This is not a score, but it is the preparation of the work. Disclosing it helps to explain the work and enables its reproduction and derivation. This is true for allographic works preparatory materials as well: music may have discarded versions, novels cut chapters and research notes.
The Creative Commons licences do not require that source (the score) be provided. It may be impractical to do so for many media, but it is ethically important to provide source material and preparatory work for autographic cultural works to help make Free Art.