On Wednesday, Sotheby's is to auction two such images, known by the names of early owners: the Constable-Hamilton Portrait and the Munro-Lenox Portrait. They are being offered by the New York Public Library as part of a broader sell-off that has drawn some criticism, and are expected to fetch, respectively, from $10 million to $15 million and $6 million to $8 million.

So what makes one Washington more valuable than another?

"When we were assessing, one couldn't help but seize on the fact that the Constable-Hamilton painting is unique," said Dara Mitchell, director of Sotheby's American paintings department. "There is not another one like it."

Painted in Philadelphia in 1797, it was commissioned by the New York merchant William Kerin Constable for Alexander Hamilton. Scholars say he may have given the work to Hamilton in gratitude for his support of a 1795 treaty that ended Britain's seizure of American ships trading with the West Indies.

The painting stayed in the Hamilton family until 1896, when it was bequeathed to the Lenox Library, which later merged with the Astor Library and the Tilden Trust to become the New York Public Library.

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