This is probably less CSS and more just HTML if I’m honest.
Do you have the image stored somewhere online (e.g. your own website)?
If you go to Account Settings >> Invoice Customisation and click on the Footer Region, you can add a bit of HTML to the Fixed PDF Footer area. This would however only show on PDFs.
You can use the same method outlined in this post, except rather than individual images, combine them into one and use it that way.
Hope that makes sense?