I need to add a image of myself in the login screen (when user credentials are asked). How can I do this?
System Preferences>Users & Groups. Click on your account name in the sidebar and you'll have the option to change your picture if you mouseover it in the right pane and click edit picture.
To edit the image associated with your login go to System Preferences > Users & Groups, click on your account in the left hand pane, click on the image in the Password tab on the right, then select Edit Image….
You also have to be certain that Display login window as List of Users is selected under Login Options of the Users & Groups preferences pane.