Bruce Jenner's daughter picked an ... interesting time to post a throwback pic of him in a crop top and cutoff shorts -- proving Kylie Jenner already understands the family biz ... promote, promote, promote.
Kylie just posted the shot of Bruce, circa 1980, showing off his Olympic Gold medal winning abs and legs. She captioned it ... "daddy throwback. #Tonight #DianeSawyer #ABC #love" -- so she's clearly reminding her 21.5 million followers to tune in for the special.
Btw ... the pic looks like it's behind the scenes from the flick, "Can't Stop the Music" -- a disco-era classic co-starring Bruce, Steve Guttenberg, and the Village People.