I have changed the icon for my primary manjaro user account using the Manjaro Settings Manager GUI. The icon updated for the manjaro user account as expected, and I noticed that it also updated the user profile icon displayed in the whisker menu. So far so good.
I would like this same icon to display next to my account name at the LightDM Greeter login screen. Is this even possible? The default user icon can be changed, but the user icons you can choose are not the same as those in the manjaro user account settings GUI.