The 'That's what she said' phrase is assumed to be of British origin, and probably dates back to the Edwardian times, where a serious conversation would suddenly take a comedic turnaround when a sexual innuendo is thrown in. In the 90's, it was popularised by Saturday Night Live 's sketch, ' Wayne's World '.
You know you want to say it but you don't, because of the feelings you don't want to hurt and the people you don't want to annoy. But enough of that. It's time to let it all go. It's time to be the pervert you always wanted to be. It's time to be honest.
So gulp down some beers, and shout out along with every picture- That's what she said!
Live your fantasy in jokes. Cheers!