Changing a User Profile Picture
By default, WordPress uses Gravatar service to display profile pictures. In short, it’s a free service that allows you to create a profile and associate a profile picture (avatar) to your email address. To change your profile picture, sign in to Gravatar and upload your image. Note that it may take an hour or so for it to show up as it propagates through the WordPress servers.
Also, there is an alternative way to change user's profile picture. You can use a plugin to manually upload the image: