I'd say those strange colourful things inside the rock piece are supposed to represent the room where Balin mixes his bombs, that's the only purpose I can think of. But whatever it is, it's nice from TLG to include something back there :)
Has anyone noticed yet that there is a shadowy Smaug on the 'Attack on Lake-town' box art? (above and a bit to the left of Bain)
Now isn't that ironic...
(On a less depressing note: to me it looks as if the left orc is shooting a fish at Tauriel "Go, herring of death!" )