Photo background for mobile devices

Replaced the images with solid colors on mobile.

@RoonyH tried to scale the image the to the container, but generally, mobile phones have greater height than width so the images look strange. Because, all the images are 700x525 i.e Width > Height

Also, fixed this :
![screen shot 2014-12-22 at 10 28 18 pm](https://cloud.githubusercontent.com/assets/2987087/5528064/14faaf06-8a2a-11e4-90c4-f414d6772f78.png)

to a lighter background (as in the desktop version)

https://www.google-melange.com/gci/task/view/google/gci2014/5798951311638528