Justin Bieber just can't quit Selena Gomez ... he lustfully posted pics of his ex-GF at an Oscar party ... even though he's 2000 miles away in Atlanta

The pic shows Gomez at the Vanity Fair party last night in West Hollywood -- where she showed up in a gold, beaded Pucci gown -- with the caption, "Most elegant princess in the world."

Could be Justin's just jonesing for his old life in L.A. -- which once included Selena?

The feeling may not be mutual ... Gomez -- who went to rehab in Jan. partly over her Bieber addiction -- has kept her distance from Justin and his new crew.