If you'd like to customise your account picture (the image you see when logging in and in many other places), follow these simple steps. Although it's not important to add an account image, it's a nice little customisation and makes things easier to identify if you're using a multiple-account system.
To begin, click the start button and select Settings:
Now, select Accounts:
The Your Info side menu option should already be highlighted and in the right side panel you should see options to Create your picture. If you've got a saved image you'd like to use, click Browse for one and select an image file. If you'd like to use your webcam to create an image, select Camera:
Note: If you're using the desktop version of Windows 10, you can quickly access the "Your Info" panel by clicking on your user icon, followed by "change account settings".