System Settings/Account Details
Password & User Account
Change your image
In order to change your image, you just need to click on the box at the left side of your username, which should contain your current image. A face selection window will pop up. There you can choose between the different images avaliables, or click on Custom image to search for an image file in your computer. Once you have chosen your desired image, press OK.
In case you are not able to edit your picture, go to Login Screen, and in the Users tab change User Image Source to "User, system" or "User".