Use post_thumbnail if it exists

This will grab the `post_thumbnail` -- if it exists -- for the post image.

If Jetpack & Photon are enabled, it will convert the image to a pretty 600x600 square.  
Why 600x600?  
It seemed like a nice size. Facebook provides **NO GUIDANCE** for what size/shape an image should be.  
Maybe we should just include the original, raw url?