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)
Nit-picky, but I'd name it `$large`... cause I foresee `$high_res` or `$high_def` being even larger and for 2x retina displays as we add more in future...