Im assuming it's Thundercracker's missing eye. And Soundwave's missing hand in the other bottom corner.
I don't wanna tear you apart or anything, but why Starscream? I think someone like Bonecrusher would fit much better, since he loves showing the other Decepticons how destructive he can be.
Starscream has a high opion of his own abilities and cares less for the health of any other Decepticon than of having power over them. Thus the grin.