Why the difference in images?
The image of W. Graham Robertson is taken from the Tate Gallery website. Sargent's painting is very dark (darker than the image that I got from the Art Renewal Center which is at my first page for Graham Robertson). The Tate's is probably a truer representation of the painting -- though you can see very little given its size.

I like the ARC image, however, because it shows more of the detail and is physically larger -- oh well -- Such is life when you're dealing with images and not the real thing.