23-year-old Sydney Elaine Leathers tells TMZ she sent the photo to Weiner in July 2012, modeling a cheeky-cut lace thong (good choice) ... in what appears to be her bathroom (bad choice).
Weiner admitted yesterday he sexted long after he resigned from Congress in 2011, but finally nipped his sexting problem in the bud last summer ... around when Leathers sent the thong photo.
Leathers says ... Weiner was very persistent in his naughty picture requests, asking her to send all sorts of naked photos ... more than 30 in total. But we're told Weiner also has a foot fetish, and regularly asked for pics of Leathers' feet in heels. Of course, this was all allegedly in exchange for pictures of his own naked ... weiner.