'Strong' Trainer Grilled Over Nude Photos

"Strong" trainer Adam Von Rothfelder has become instantly famous as the breakout star of the new show ... so like half of Hollywood it's not a big shocker nude photos have surfaced.

Turns out Adam posed nude for an art project and the pics are ... impressive. The honchos on "Strong" were a little upset ... not because of the pics but because Adam didn't fess up before he was hired.

We're told Adam explained he didn't think it was a big deal because it's not pornographic. Producers saw it his way, so he won't be punished. It helps he's also a big reason people are watching.

Dave Broome -- the creator of 'Biggest Loser' and "Strong" -- is a big, big fan of Adam's work ... telling us, "I am actually rather envious."