This fascinating painting is known to be a piece of a harpsichord lid, notably the flap covering the keyboard.
Salvator Rosa, other than an extremely talented draughtsman and painter, was also a poet and a musician. I had the fortune of holding in my hands a manuscript of vocal music (Paris, Bibliotheque National, Music Department) that most likely belonged to him, as some of the textual compositions may have been written by the artist.
This is all well known. Here is some literature and a recent video on the music behind the skull. Enjoy!
Margaret Murata, Dr Burney Bought a Music Book..., The Journal of Musicology, Vol. 17, No. 1, A Birthday Tableau for H. Colin Slim (Winter, 1999), pp. 76-111 (36 pages), Published By: University of California Press