Police chief Robert Nivakoff said the alleged conduct, if verified, was "morally reprehensible."
The 26-year-old woman said the 40-year-old Mabey, who is married, showed her the picture Wednesday after he made a pass at her, and she told him he was too old for her.
Mabey had stopped her for talking on a cell phone while driving.
Police union President Sgt. Joseph Kennedy said Mabey, the city's 2006 Officer of the Year, was a diligent officer and was cooperating with the investigation.
