If you have signed with Google or Facebook, your picture will match that used on your Google+ or Facebook profiles.
If you created an account or signed up with Office 365, your profile picture is taken from Gravatar
using your email address. To change it, simply sign in to Gravatar (or make an account if you don’t already have one) and upload a new profile picture. Gravatar is very widely used across the internet (and is even used on the website you're reading this article on) so it's worth signing up for.