Hi, my name is*****'m a certified appraiser, and I'd be happy to help.
You are referring to Miss Juliana Willoughby, painted by George Romney in 1781. The original of the painting is held at the National Gallery of Art.
Your item is a printed copy, that would be sold in a museum gift shop.
Unfortunately, as tens of thousands of these are printed and sold, the resale value is no more than $10-$20.
I'm sorry for any disappointment.
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