More like her ribcase is just messed up. I'm more surprise to see that she has a tail. She pretty much look like a xenomorph here.
The tail is a surprise, but double checking with the other pic he made of her, it seems like we´re just seeing that thing that comes out of the middle of her chest, because her boobs are translucent.
It's the transparency and part of the armor that covers her chest I think. Still this looks really cool and that fits her design surprisingly well in my opinion.
This image is part of kodama's "Effigy" art book, if you want the entire thing, here's a link: http://imgur.com/gallery/1VoJu (not uploaded or scanned by me, just sharing)