Hey jali, Its quite tough to find good referance pics for shoulder muscle definiton.. i mean those generic anatomy models have pissy little muscles.. i found a pic that might come in handy for you tho, so ill post it here for you... dont get the wrong idea or nothing, this pic might help you get some more non-linear shape to those muscles i hope it helps dude

Take it easy chief..

-Ruff
(PS.. its not me lol)