Responsive images for showcase elements in the body

We should upgrade our images in the body, since the 620 standard width would be scaled. Quality is improved a bit.

Before:
![before](https://cloud.githubusercontent.com/assets/4549425/3055354/93ef87a4-e1c0-11e3-869c-eb7367c995f3.png)

After:
![after](https://cloud.githubusercontent.com/assets/4549425/3055355/960d8130-e1c0-11e3-88ce-f3bb6cf219ae.png)