Short answer. The painter's side. Where two things are joined and neither is obviously principal, the Code prefers the more valuable and then the bulkier — but for painting, sculpture, writing, print and engraving Article 468 settles it outright: the canvas, board, stone or paper counts as the accessory thing.
What the law says
In painting and sculpture, writings, printed matter, engraving and lithographs, the board, metal, stone, canvas, paper or parchment shall be deemed the accessory thing.
Civil Code, Article 468 — Principal by Value or Volume. Read the full provision →
The default test, and why it is switched off here
Ordinarily, when two things belonging to different owners are joined so that they cannot be separated without injury, the law looks for the principal thing and treats the other as an accessory that follows it. Where that cannot be worked out, Article 468 first prefers the thing of the greater value, and only between things of equal value does it fall back on the greater volume. For artistic and printed work that ranking is abandoned. The article does not weigh a canvas against a painting at all — it declares the support to be the accessory whatever the two are worth.
What the rule is really recognising
That the value in the finished object is the work, not the material it sits on. A blank canvas is a commodity; once painted it cannot be recovered as a canvas, and stripping the paint to return it would destroy the only thing anyone cares about. The same reasoning covers the stone under a sculpture and the paper under an engraving. Note how wide the list runs — board, metal, stone, canvas, paper or parchment — so the outcome does not turn on the medium being a traditional artist's one.
Being the accessory is not the same as getting nothing
This is the assumption most readers arrive with and it is wrong. The article answers one question only: which of the two things is principal, and therefore whose ownership absorbs the other. It does not say the canvas owner walks away empty-handed. What he is entitled to depends on how the two materials came to be joined and, in particular, on whether the person who did the joining acted in good faith — the accession rules deal with indemnity separately, and a painter who used a canvas he knew was not his stands very differently from one who did not.
Ownership of the object is not ownership of the image
Worth separating before any demand is made. This provision allocates the physical thing. Rights to reproduce, exhibit or licence the image are governed by a different body of law entirely and are not decided by who ends up holding the panel. Disputes of this kind — commissioned work, a print shop's stock, materials supplied by a patron — often turn out to be about the second question rather than the first, and the written terms of the commission or supply arrangement usually matter more to the outcome than the rule on accession does.