You can define a photo of yourself which is displayed on the login screen. Choose SystemPreferencesAbout Me . The following window will be displayed:

The About your username window

In the upper left corner of this window, there is a button which is still empty. Click on this button to open a file chooser. You can choose now from a lot of pre-installed user faces. If you want to use a real photo of yourself, navigate to the desired file and select it. Once you have taken your choice, click on Open. The file chooser disappears and your picture is now displayed on the button.