Campaign Tile image: data-src-large attribute

Adds the `768x768` image URL as a `data-src-large` attribute to allow fenders to begin fix for #3267.

Note: This will only appear upon cache clear, as the DS Image cache will need to be truncated to see the new attributes.

Output:
![screen shot 2014-11-20 at 12 50 00 pm](https://cloud.githubusercontent.com/assets/1236811/5129897/6ed86b4c-70b4-11e4-8c71-21032fd1a983.png)
The ratio corresponds to the field to pull the file from on the Image node.  There is Landscape, Square, Thumbnail, and now Video, too.  This could easily be swapped out for the Square field, I was basically following what was already in there.  For Cover Images, each Image node needs both a Square and Landscape file uploaded in order to display, so .. we can change this to Square field.  I can do in the next round of changes.