Sidebar title image for Twenty Twelve

Hey

I would like to customize my site a bit. I am usin Twenty Twelve theme and have made all my edits in a child theme.
What I am trying to achieve at the moment is to get that picture (https://helevalgus.com/wp-content/uploads/2015/09/watercolorbanner250v2.png) behind the sidebar title text, but adding background-image: url; to css doesn’t seem to do the trick. Sadly.
Any idea how to achieve this?

I would also like to get a custom image background to the menu navigation.

Thanks in advance.