Complex sentence: One figure who was thought acceptable was George Nassau Clavering-Cowper, 3rd Earl Cowper, who is shown on the left of the painting contemplating the aesthetic virtues of the "Niccolini-Cowper Madonna"-which Zoffany eventually sold to him.
Split into simpler sentences: One figure who was thought acceptable was George Nassau Clavering-Cowper, 3rd Earl Cowper. Although Cowper is shown on the left of the painting contemplating the aesthetic virtues of the "Niccolini-Cowper Madonna"-which Zoffany eventually sold to him.