I would guess that taking the pics and making a sort of a game of it helps them de-personalize the enemy and deal with the carnage. Revenge and morbid humor make sense too. People slow down for car wrecks right.
Perhaps the Pentegon doesn't know about this site. What is the website do you know? Could these pictures had been posted by someone other that a soldier, using this for propagada purposes? How can one prove that American forces took or posted these pictures?