There is a 1/4 inch bleed on this that should be cropped off for mock-ups/reMIXes ;^)

In the upper left corner would be a small icon to represent the image choice used on the front of the postcard (could be the image itself smaller or even the users profile icon) alongside a short copy blurb with the title, a short and sweet description of the image underscoring how many user contributions led to the collaborated image.