For images depicting a scene that includes a piece of artwork, i.e. an actual painting (noun), such as on canvas, or framed on a wall.
If a character in the image is shown engaged in the action of painting (verb), use painting instead.
When visible artwork is being actively painted (e.g. post #1255502), both tags are applicable.
When something is being actively painted, but the artwork is not visible (e.g. post #1680283), do not use this tag.
Also consider tags such as easel and canvas (object) where applicable.