Lubuntu is a lightweight and great operating system, unfortunately it doesn’t include a default theme that is easy for changing the default avatar. I found the following trick that might help.
First, let’s find a favorite photo that you want to set up as an avatar, then use one of the website like this to convert the photo into an icon (.ico) file: https://icoconvert.com/
Don’t worry for the .ico, it’s not a very low favicon.ico you often see. Next we need to copy this file to the following location:
/usr/share/sddm/faces/ with the name like
YOUR_NAME is your linux login. And that’s it, you can then restart your computer for a check.
# please change YOUR_NAME and avatar.ico according to your local
sudo cp Downloads/avatar.ico /usr/share/sddm/faces/YOUR_NAME.face.icon