In the scene where Sherlock is talking to Mycroft after his extraction from Serbia, they are in a dark office. My presumption is that this would some kind of office belonging to Mycroft, and not necessarily one he commandeered. Although we don't see the office before season three, the presence of the painting in the scene (amidst the general darkness) is somewhat focal and pronounced.
Who is the person that is the subject of the painting, and is it an otherwise historical painting? If it is not a specific individual, is there a resource that helps (non-British) viewers better understand the various iconography and artwork frequently accompanying scenes with Mycroft?
Here is a screenshot: