Sherlock Season 4: 20 Easter Eggs & References In 'The Final Problem'
20. Mycroft's Portrait
During Sherlock and John's elaborate prank on Mycroft at the start of the episode, he becomes spooked after the portraits on the walls weep blood.
One of these portraits looks a lot like Mark Gatiss in Elizabethan dress, presumably intended to be an ancestor of the Holmes family. This recalls a plot point in The Hound of the Baskervilles. Holmes realises that Jack Stapleton is in fact a member of the Baskerville family due to his uncanny resemblance to one of the portraits hanging in Baskerville Hall.
If you were wondering who the man in the clown suit and the dwarf woman dressed as a child in this scene are, Sherlock tells them to collect their money from Wiggins and "don't spend it all in one drug den." Presumably, then, they are members of his Homeless Network.