Talk about your tempest in a teacup stories. Patriots tightend Rob Gronkowski being photographed with a porn star and apologizing for it has become subject dejure. Here's my take on the situation.
Fist off - he didn't tweet the pictures - Bibi Jones tweeted the pictures. Second my understanding is that he apologized to Mr. Kraft in passing. More sorry for the attention the story is getting then for the pictures being posted I would guess.
I do think that if he was going around wearing his uniform jersey then he should apologize for that. He's not in high school where wearing the jersey on a Friday night was thought cool by some. If he was wearing his own jersey he should apologize for being a dork (although being with Bibi Jones erases much of that dorkiness if that is the case).