It's just Photoshopped pictures that contain three key elements:  A sandwich or sandwiches, a waterfall and Tom Selleck.

An example:

Describing the rest of the pictures won't do them justice.  This is one of those things you have to see for yourself.  Check it out here:

http://selleckwaterfallsandwich.tumblr.com/

(And don't forget to scroll all the way down and play the official Selleck Waterfall Sandwich theme song!)