Sets an image height for mktplace screenshots

Prevents the page from bouncing up
and down when a portlet has screenshots with
different heights.

The max height of an image was already set to 20em.  This makes the container 20em as well and centers any image that does not fit.

Before the image would be the same height as the image, producing a 'boucing image affect' (https://coderwall.com/p/uf2pka) for images with different height. 

Now:

![image](https://cloud.githubusercontent.com/assets/5521429/3292914/ec695738-f594-11e3-8578-794f9f324b2a.png)

![image](https://cloud.githubusercontent.com/assets/5521429/3292918/f3a14204-f594-11e3-82a8-95585a6b7ad8.png)