Make article profile image mobile first

I seemed to have ignored the mobile breakpoint when refactoring the profile images last week.

/cc @paulrobertlloyd 


### before
![screenshot from 2014-03-17 17 25 54](https://f.cloud.github.com/assets/80089/2439085/75a29e26-adf9-11e3-87b3-323ab63f6852.png)

### after
![screenshot from 2014-03-17 17 26 19](https://f.cloud.github.com/assets/80089/2439090/7e86d566-adf9-11e3-96fc-a447b0c04cf1.png)