I've done a studies of fragments of master works with graphite and charcoal before, and I find few things much more difficult to achieve using a digital medium: base blocking and edges being the main ones. I actually just begin working with the values straight away, and loose accuracy..
These first studies are from some great photos:
Something about that big round shape caught my eye. The staircase is there really just to give that dark tone in the back, to create stronger emphasis.
The hats - one black and the other white - they're like a game, my eye just gets attracted to it.