the easiest way to change the icon of not just the login screen, but globaly.
#how to get
to get the sript you first have to install git
sudo apt install git
now you just clone the project
git clone https://github.com/thayeeboi890/ubuntu-login-screen-logo-changer.git
enter the directory
cd ubuntu-login-screen-logo-changer
make it executable
chmod +x changeubuntuloginlogo.sh
and now you just have to run it
sudo ./changeubuntuloginlogo.sh
now you enter the location of the picture
my picture is located at /usr/share/plymouth/ubuntu-logo.png so I enter,
and now say yes
if you want to you can restart gdm
if not just put 'n' and you are finished