Self portrait with a Harp, Rose Adélaïde Ducreux
In the Metropolitan Museum of Art. Use the download arrow under the image for larger version.
Rose Adélaïde Ducreux, who studied with her father, painter Joseph Ducreux, here portrays herself with a harp and in a luxuriously finessed gown that dominates the work. I suspect that, like many self portraits, it was meant as a display of the painter’s skill to prospective portrait commissions.
Link: Self portrait with a Harp, Met Museum
I understand the term Masterpiece originally was meant as a sort of demo or showpiece to prove you could handle all manner of subject matter. A self-portrait would let you prove you can do portraits.