Standard you will see a computer icon when you login. If you want to change this, you need to go to Preferences / LightDM Appearance. This is just a graphical application to will pass the information to a file that is in /etc/lightdm/ folder.
Changing that file will also work.
Then we are going to find a real logo/avatar and I will try to display it on the login screen. We are retracing all the steps I did yesterday. Figuring out how to display our avatar.
We started looking in the wiki and then finding our way around ArchLabs.