Here we see Sherlock in an ailing state facing mortality in the form of the skull, which takes up most of the foreground. Sherlock has remarked that the skull (death) is like a friend to him, but here he can barely look death in the eye. However, Sherlock and death are connected by sharing the same, pasty clay hue (from dust we came), the one variable separating Sherlock and leaving him clinging to life being his dark collared shirt. This is a brilliant reversal of the typical representation of light and dark as signifiers for life and death.
Sherlock’s multiple chins indicate his many near-death experiences (much like scars, similar to John’s fractured psychosomatic psyche), or perhaps, like a cat, his remaining lives (the number of chins Sherlock has in between shots radically changes, however, so this may be a stretch).
Moffat’s choice to include the youtube bar in the framing of the shot is a precursor to the finale of the series where Moriarty reveals that he has been watching Sherlock the entire time, as if he were a highly entertaining internet video about cats or people hurting themselves violently. We are witnessing Sherlock from Moriarty’s gaze then—this is a subsersive technique to make us, the audience, doubt Sherlock Holmes and sympathize with Moriarty at the end of the second series; however, remarkably, we have outfoxed Moffat and have rebelled with the “I Believe in Sherlock Holmes” movement.