Lindsay Lohan Photographed Pounding Tequila

Sources close to Lindsay tell us .. the actress was hanging out in Venice last week when a friend-of-a-friend asked if he could take a picture with LiLo.

When she said, "yes" -- we're told the guy handed her the bottle and told her to smile! Lindsay is telling friends she didn't take ONE single sip of alcohol that night and put the bottle back down after the photo.