One picture alone should not be enough to cement her as a good Mary Jane.
It's not the one picture, but I will admit I flip flop back and forth on Shailene. I was jazzed when she was cast based on having seen her in The Descendants and really liking her. Then when I saw the set pics from TASM2 I was worried to be honest, she looked possibly miscast to me. Now I've gravitated back towards wanting to see her in the role, but I do have concerns. I just want her to be a little more glammed up than what she looked like in the released pics, and I've seen many photos of her to suggest she can be a very pretty MJ for the films.
This is the pic I was referring to in Details btw (at the time the article was written, she hadn't yet been dropped from TASM2).
Whoever they get, she has to be a great actress who can deliver a performance that allows audiences to fall for her and accept Peter moving on from Gwen to her. Which is my biggest concern about many of the pretty actresses bandied around, I don't believe they have the acting chops to pull that off.