Clip background image previews that overflow bounding box (bug 1029822)

## Before:
![screen shot 2014-06-24 at 4 47 21 pm](https://cloud.githubusercontent.com/assets/23885/3378871/55ba486e-fbe9-11e3-93d5-477f40f035c3.png)

## After:
![screen shot 2014-06-24 at 4 47 25 pm](https://cloud.githubusercontent.com/assets/23885/3378876/5cb7b11a-fbe9-11e3-8010-a9d9b12a1b11.png)