The use of the blue color is interesting and certainly helps sort of tie every image together and brings everything to tie in with the finale at the pool. Also, as it is impossible to capture every single important scene and the ones chosen are as representative of the episode as any. The fact that Moriarty is missing is an interesting choice. When it doesn't make sense to begin with, you gradually realize that his presence is portrayed in a clever way, since, of course, he has a hand in all those scenes, being only revealed at the end. In this way, the art is a clever teaser poster that makes you curious to find out more. Finally, I have to comment in the expression of Sherlock and John which are very well done and captured perfectly. They draw attention the details and makes the observer aware of the artist's talent.
Again, this looks like the cover to a graphic novel I will now never have (grumbles…) but well done. The blue and black sets a somber tone, and the light on the water in the last panel is incredible. There's just enough red thrown in for contrast that it catches our eyes without ever being distracting.
As a fairly new Sherlockian all I can say is...I love this. The choice of scenes is very appropriate, and tells the story of the episode very well. And the colour scheme is very nice, having the blue of the pool translate through the other scenes gives this piece a greate sense of continuity. I even love the pool! I almost didn't realise it wasn't a photo!