Sir Henry Raeburn was a Scottish portrait painter. You can read more about him in his Wikipedia entry here.
I have always loved this picture, because I find it amusing to think that a minister would allow his portrait to be painted while he is skating. And he is so very solemn and upright. I love the contrast between the dark black of the minister's clothes and the greenish background colors. The diagonal forward slant of his skating body. Though he seems so very proper, there must be a hint of fun in him--or we wouldn't see him skating, would we? He could have picked the standard "sitting for a portrait pose."