I am myself such an owner; some time ago I bought a painting at a flea market that illustrates a scene from Macbeth. Macbeth stands in a gloomy interior before the three witches; beside them is a naked young man wearing a crown, and behind Macbeth hovers a woman who probably represents Hecuba. The scene is luridly illuminated by the fire beneath the witches' cauldron, and the picture has an attendant owl and some kind of nasty little beast scuttling by a seated witch. The scene depicts the second visit of Macbeth to the witches (Act IV, scene i), but that is almost all I know about the painting. Various exhibition catalogs describe several such paintings by various artists working in the late eighteenth and early nineteenth centuries, and the canvas is unsigned.

I sought help in identifying the painting from Professor William Pressly of the University of Maryland; he is the author of A Catalogue of Paintings in the Folger Shakespeare Library and an authority on British painting. He sent me several xeroxed pages from Esther Dotson's dissertation, Shakespeare Illustrated, which has an appendix entitled "Subjects from Shakespeare in the Exhibits of the Artists' Societies from Their Inception Through 1830." The list compiled by Dotson suggests several possible artists for the painting:

William Dawes, Macbeth at the Cauldron (1760)
George K. Ralph, Macbeth Meeting the Witches at the Cauldron (1790)
P. Maquignon, Macbeth Consulting the Witches (1793)
Henry Singleton, Vision of the Infant Monarch (1808)
Joseph Barney, Macbeth Entering the Witches' Cave (1809)
Henry Singleton, Macbeth and the Vision of the Kings (1814)
Joseph Barney, Macbeth Entering the Witches' Cave (1820)

Here the matter rests for the time being.