The artwork, painted by Jacques-Louis David in 1813 when Britain and Prussia were threatening to occupy France, shows Napoleon I pledging to defend the country from invasion, wearing his national guard uniform to do so.
He is posing, as was his habit, with his right hand in his waistcoat.
Thought to be a copy, the painting was sold in 2005 to a New York private collector for around USD 24,000.
He compared it to copies and other David paintings.
"Although the painting is signed with David's genuine signature, the cleaning revealed the word Rouget and the date 1813 appeared in the underpaint," Lee said.
Georges Rouget was David's preferred assistant for almost 10 years, and had a role in putting images on the canvas.
"Some collectors or museums might be put off by having two names on the canvas -- but in many ways that is proof that it is an authentic product of David's working process."
"Our collection policy is for Scottish arts and Scottish items so we don't collect work by European artists," an RSA spokesman told AFP.
"It was valued by a number of auction houses, and experts looked at it and it was given the status of being a copy.
