by your own admission, he's your ex, why were you snooping at the contents of his phone, prying into his private life, you're no longer the centre of his universe.
Is it your idea that men looking at other women is cheating, but women looking at other men isn't cheating? Women do exactly the same thing, eye up the opposite sex. A good few years ago I queued for, five hours to get Barry Manilow tickets, my wife was a big fan. We went, and it was a good show, despite the hundreds of screaming, middle-aged, married women, I suppose they weren't cheating.
The nude woman on his phone is no more significant than a copy of Playboy, and none of your business anyway, he's your ex, remember.