Main image

I am using your feed for pinterest at the moment. maybe more in the near future.
I have a problem with the main image.
As I am using a app from BDroppy(brandsdistribution) and the images are provided through url. so they are not actually saved on my server.
How can i add these images to the feed. as I do not see option.

The page I need help with: [log in to see the link]