How to change account profile picture?
It is under System Settings -> User Accounts, look at the mouse pointer in this screenshot:
Go to settings, User Accounts, and click "Unlock" on the top right, then choose the account you wish to modify. Then, click the current avatar and choose another one.
Ubuntu 18.04
"Settings" -> "details" (scroll to the bottom) -> "users".
or just from the menu (where is the power button), click on your username -> "Account Settings".