Comment 1 for bug 1824645

This feature already exists (at least if I understand your request correctly.
We called it "badges" and they are what identifies the session in the session menu in the top panel - in case the user has multiple sessions available, just create a pull request adding your logo here:
https://git.launchpad.net/lightdm-gtk-greeter/tree/data/badges

Obviously that's visually different from just having a "watermark" logo somewhere in the screen, but then again I'm not sure that's a very essential feature...