Change to onDeck poster img CSS

I made a small change to the onDeck poster image CSS so it doesn't look stretched.

before:
![before](https://cloud.githubusercontent.com/assets/6559052/2830385/51714eb8-cfae-11e3-844b-c1da00e427e2.PNG)

after:
![after](https://cloud.githubusercontent.com/assets/6559052/2830390/57b8e75e-cfae-11e3-90cc-d3c644335708.PNG)