I’d say it cleaner and more versatile to make a theme component using the plugin outlet, plus you’ll learn useful things if you want to customize further your installation. 
Using a pseudo element from existing HTML to add your image will work and it’s a valid way to achieve this. 
That said, it depends on what result you want exactly.
For example, if the goal is simply to have a background image at the bottom or top of the login buttons block with no additional requirements, then CSS could be perfectly fine.