use image scale in context

This fixes the blurry message bubbles.

Old:

![screen shot 2014-09-17 at 2 27 43 pm](https://cloud.githubusercontent.com/assets/31402/4311733/b70d439a-3eb1-11e4-9845-5a9d978b9afd.png)

New:

![screen shot 2014-09-17 at 2 28 15 pm](https://cloud.githubusercontent.com/assets/31402/4311734/bbb93624-3eb1-11e4-8857-415650539ce3.png)