The options I'm given don't match the FAQ. There is no option for me to change my profile picture under account details. After email, account type and date of birth there is no option for me to change my profile pic.
Click on the big green 'N' in your post. This will take you to your public facing profile where you'll see the big 'N' again, in the top left hand corner. Cursor over it and the word 'edit' will be revealed in white letters. Click on it and it will take you to the place where you can upload a profile pic.
It seems you don't have the option between account type and date of birth, to change you avatar. I don't know why that is, but a guess is, it might be because you have not chosen one yet. So I suggest trying mia-me 's method, and if that doesn't work, write here again.