There's more to that Jay Cutler butt pic than meets the (brown) eye ... so says Kristin Cavallari!
Turns out, the famous Jay Butt-ler pic was taken during an outing to a nude beach during a romantic trip to Mexico back in March.
Cavallari told the story to our friends at TooFab ... saying they got caught up in the moment and figured, "When in Rome ..."