Jon Gosselin allegedly slammed his departing girlfriend Hailey Glassman against the wall of their Manhattan apartment after he saw a photo of her kissing “Celebrity Boxing” promoter Damon Feldman. “That picture started the whole thing, a picture of her kissing another man,” a source told The Post’s Jamie Schram. Earlier, Glassman had “found out he was cheating on her. She told him it was over,” the source said. Glassman filed a harassment complaint with the NYPD, saying “she was pushed against [a] wall” on Dec. 20 and verbally abused by Gosselin, a charge under investigation by cops. “The document speaks for itself,” said Glassman’s lawyer, Stephanie Ovadia. “Jon is a lover, not a fighter,” said Gosselin’s lawyer, Mark Heller. “He’s been rumored to steal a kiss here and there, but he has never been accused of landing a sucker punch.”