When congressman Anthony Weiner said, that he could not "say with certitude" whether the lewd picture twitted to a college girl was of him, Congressman Weiner effectively admitted that (1) it looks like his underwear; (2) he has taken pictures of himself turgid or someone has done it for him. Otherwise, he would absolutely know that the picture was not him.
Accordingly, guilty or innocent of that particular tweet, he knows he cannot absolutely deny that the picture was of him and effectively admits that it could be.
Furthermore, if his account was hacked or pranked, he should have immediately reported it to the FBI, which is easily done online. Or he could still do so. Why not? Because if he actually sent the tweet, the false report would be a crime.
To me the evidence on Weinergate is that the wiener in question is Weiner's.