One of the pictures shows a girl and boy - who look similar to Justin and Selena and appear to be wearing few clothes - lying down together, while another features just a pair of holding hands.
The images were quickly removed from the photo sharing app and it's still unclear whether someone hacked into 18-year-old Selena's account and posted the lookalike snaps.
Teen heartthrob Justin, 17, recently revealed that he had every intention to keep up his squeaky clean image.
'I don't want to start singing about things like sex, drugs and swearing,' Justin told America's V magazine.
'I'm into love, and maybe I'll get more into making love when I'm older.
'But I want to be someone who is respected by everybody.