Then came perhaps the least convincing denial in the history of such scandals, when Mr Weiner said he could not say "with certitude" that he was not the one pictured.
He dismissed the need to investigate the matter further with the comment: "I'm not really sure it rises, no pun intended, to that level."
But the media interest continued, with many pundits questioning why the supposedly guiltless politician was not present at Sunday's Celebrate Israel Parade in New York, an event he has always attended in the past.
More than 30,000 people attended the march, including Mayor Michael Bloomberg, making the absence of one of Israel's strongest congressional supporters the subject of great debate.
On Monday Mr Weiner cracked. "To be clear, the picture was of me, and I sent it," he said. "This was me doing a dumb thing, and doing it repeatedly, and lying about it," he said.
He said he had intended to send the woman a jokey private message and once he realised he had exposed the photograph to the world, he panicked.
He said that although he had "exchanged messages and photos of an explicit nature with about six women over the last three years", including after he got married last year, he had never had any physical contact with the women.
"I'm deeply ashamed of my terrible judgment and actions," he said.
Mr Weiner said he would not resign, although House Minority Leader Nancy Pelosi said she would seek an investigation by the House Ethics committee.
Another Jewish politician, also once considered a rising star of the Democrats, stopped short of calling for Mr Weiner to resign but said it was "cringeworthy" and not a moment he should be proud of.
"Believe me, I know, I've been there," added former New York governor Eliot Spitzer, whose involvement with a $1,000-an-hour callgirl cost him his political career.
This is not the first time Mr Weiner has made headlines by posting a photograph on Twitter. In February, when he reached 10,000 followers on the site, he showed them a snapshot of himself from his barmitzvah in 1977.
