"Anthony's accounts were obviously hacked," Arnold said. "He doesn't know the person named by the hacker, and we will be consulting on what steps to take next."
The photo showed a man's bulging underpants.
It first was reported Saturday by BigGovernment.com, a website run by conservative commentator Andrew Breitbart. The site said the photo was tweeted to a Seattle woman.
The photo was quickly deleted.
Weiner later joked about the account hacking on Twitter, asking whether his kitchen blender would be next to "attack" him.
Weiner represents New York's 9th District, which covers parts of Queens and Brooklyn, where he was born and raised. He failed in a 2005 bid for the Democratic nomination for mayor of the city but still is widely considered a contender for the office.
He has worked on issues including public housing, homeland security and energy for his constituents.
Weiner, 46, is married to Huma Abedin, an aide to Secretary of State Hillary Clinton.