This miniature of Marie-Antoinette represents that irritating internet phenomenon, an image which is endlessly reproduced on Tumblr, Pinterest and various blogs, but with no real information about its provenance. The original post, I think, was the one listed below on the Grand ladies website.
The picture is annotated here as "Marie-Antoinette wearing uncolored hair by François Dumont (Boris Wilnitsky)", so we can probably safely conclude that it was a piece sold by the Viennese Fine Art dealer Boris Wilnitsky which has long since disappeared from his website. Dumont usually signed his work, so there is no reason to doubt the ascription. (Boris Wilnitsky has a number of current Dumonts, all informatively documented).
Obviously the striking feature of this particular portrait is Marie-Antoinette's surprisingly luxuriant shock of red hair. So how true was this to life? [Okay, I know, trivia doesn't come much more trivial....]
Dumont, who was official Court miniaturist from 1786, painted miniature portraits of the Queen over a long period, from about 1777 until almost the end of her life, so presumably he knew what colour her hair was. On the other hand, he himself lived long into the Restoration and his oeuvre is estimated at c.2400 works, so there was bound to be a bit of variation in quality. The Louis XVI, though realistically chubby, is not particularly convincing.
I suspect the thickness and redness of Marie-Antoinette's hair is exaggerated for aesthetic effect. There are no dimensions on any of the postings but the setting is described as a locket, so I wonder if the portraits are tiny? The very bright red hair, echoed in the skin tones and complemented by dull greens and blues, provides a strong simple palette for a very small scale painting.
Generally Dumont repeated the same poses with minor variations in different portraits, but there is nothing else remotely comparable to this picture on the internet, which, apart from the hair colour, is unusual in its informality. The picture remains, therefore, something of a mystery.
A definitive answer on Marie-Antoinette's hair colour is actually quite difficult to find. In the vast majority of portraits she wears hair-powder; where she does not, the hair does indeed have a ginger tinge - but how dark is hard to assess, given the vagaries of reproduction and the possible discoloration of the original oils. Unfortunately, the Mémoires secrets of Lafont d'Aussone, the source cited by Antonia Fraser and others, described the young Dauphine as "ash blond" (blond-cendres), certainly not a red shade by any definition. On the other hand, Madame du Barry referred to her disparagingly as "La Petite Rousse". The majority of modern commentators have gone for "strawberry blond", which I think is a dark gingery blond shade. The numerous samples which survive substantiate this; Le forum de Marie-Antoinette has a thoroughgoing selection of reproductions; here is just one example, owned by the British Museum:
Reproduction of the miniature on the Grand ladies website