Miniature of Maj. Pitcairn
The miniature is a copy (probably 19th century) of a late 18th century miniature in the possession of the Pitcairn family, and reproduced in Constance Pitcairn's book, 'The Fife Pitcairns', 1905. The hairstyle, high coat collar and style of epaulette are of the 1780s-90s, so it clearly cannot be a contemporary likeness of Major John Pitcairn. It may be an idealized posthumous portrait, derived from the representation of him in Trumbull's painting of the Battle of Bunker Hill. Trumbull used Major Pitcairn's son, Dr. David Pitcairn, who greatly resembled him, as his living model for Major Pitcairn when he was painting his famous painting in London in 1785-6.
Another possibility is that it is Major John Pitcairn's son, Thomas Pitcairn (1755-97), who carried his father from the battlefield. Thomas had become an infantry Colonel by the time of his death, aged 42, at Chatham, after a long illness. There was a strong physical resemblance between all the male Pitcairns. A likeness of one of them in uniform could easily have been assumed by later generations to be the most famous one, especially if the miniature had depicted Thomas when he held the rank of Major. Such a portrait of 'Major Pitcairn' would readily have been assumed to be of the father, not the son.