The photo, which showed a man's bulging underwear, was sent from his Twitter account on Friday to a 21-year-old female college student in Washington State.
"Last Friday night I tweeted a photo of myself that I intended to send as a direct message as part of a joke to a woman in Seattle," Weiner said. "Once I realized that I had posted to Twitter, I panicked and I took it down and said that I had been hacked. I then continued with that story, to stick to that story, which was a hugely regrettable mistake."
The picture was quickly deleted but posted long enough for others to see. Weiner has more than 54,000 Twitter followers.
Weiner said that he was sorry but was not planning to resign.
Soon after the confession, House Minority Leader Nancy Pelosi called for an Ethics Committee investigation to determine if any official resources were used in his Twitter postings and whether any House rules were violated.
"I am deeply disappointed and saddened about this situation; for Anthony's wife, Huma, his family, his staff and his constituents," Pelosi said in a statement.
Weiner said he will cooperate with the ethics probe.
He also admitted during the news conference that he had "inappropriate" conversations with six women before and after getting married, but that he never personally met any of the women he corresponded with online or sometimes on the phone. He said he was not sure of their ages.
The congressman also said he had never had sex outside of his marriage.
Weiner also admitted sending photos to another woman, 26 year-old Megan Broussard, who spoke to ABC's Chris Cuomo.
"He was eager to hear if I wanted him or thought he was attractive," Broussard said. Asked how much of it was sex talk, Broussard said, "he would attempt it all the time."
She also said she received an explicit photo of a man's exposed private parts.
The Weiner photos were posted on biggovernment.com by conservative blogger Andrew Breitbart, who commandeered the microphone before Weiner took the podium at the news conference.
The 46-year-old Democrat has been considering a run for mayor of New York City. He's known for his humor and sometimes combative style.
The Associated Press contributed to this story.