Ensure the image inside mega-full item is correctly cropped

It was previously being squashed into the space rather than a center crop.

#### Before
![google chrome](https://cloud.githubusercontent.com/assets/80089/4403329/752d6464-44a1-11e4-9872-106f4def580d.png)

#### After
![google chrome](https://cloud.githubusercontent.com/assets/80089/4403332/77c4a4f8-44a1-11e4-84e1-95a48ed986ef.png)


/cc @sndrs @robertberry @kungpochicken