Add a `picture` component

I'm opening this PR early to try get some discussion happening around the implementation I've gone for.

![picture-component](https://cloud.githubusercontent.com/assets/708296/3890508/75a031c8-221f-11e4-8da1-dc53dfc7b510.png)

Yeah, I was thinking of doing that... might come bug you about what the best way is to do that.