To change the logo of the login page an image can be displayed, but also an Apex-icon can be used.
In the attributes of the login-region undes Appearence add name of icon.
So : Appearance – Icon – fa-pie-chart
And other option is to use a custom image. Define a class in CSS which gets the image and use this class as icon name
So : Appearance – Icon – apimg
background-size: 205px 109px;