There is an option to change the user account profile picture in elementary OS, except that there is high chance of missing it, because it is not specifically mentioned that there is something with the profile picture.
Launch ‘System Settings’ from the plank or whatever the way you do.
Click on ‘User Accounts’.
Select the user account for which you want to change the picture from the left pane. Then click on the icon near to the user name in the right pane.
You will have to now click ‘Select from file’ and browse and select a profile picture. It is recommended to use a square image. Even if you select a rectangular image, the tool will ask use to crop it to square area.
Click ‘Change Avatar’ and you should be done.
Lock the screen to see your new profile picture.
Note that you can’t change the default profile picture of the Guest account.