A girl with no pictures of her that expose much/anything below the neckline--the insinuation being the presence of weight or general body-image issues. The term can be altered for a male of the same nature.
A: Hey, how come this girl doesn't have any pictures of her whole body? All her pictures are just different angles on her face.
B: That one girl Tommy just started dating? Yeah, she's a neck-up girl
A: Aw, man. Good luck, Tommy.