In Windows 10 adding or changing account picture is very easy. Use below quick steps to set your favorite picture as account picture.

1. Open Windows 'Settings' App from start menu or by typing 'Settings' in cortana search box, and selecting 'Settings' Windows App.

   

2. Navigate to 'Accounts' -> 'Your account'

   

3. Click on 'browse' button in the window and select your favourite picture to set as new account picture.
4. If you want to set your own snap as account picture, click on 'Camera' button in 'Create your picture' option, which will open camera App. Click on camera button in app to take your snap for new account picture. After snap is taken, click on Apply button to set your new snap as new account picture