TwentyTwelve header img – turn into bg img for content

Hello,
I’m new to wordpress and have limited css php knowledge. I am using Twentytwelve and have a header image up (https://www.coringrillo.com/). What I need is to be able to edit the header space exactly like a wordpress page can be edited.
Purpose: I want to make the header image, a background image in the same position and be able to put text on it and insert the plugin code for “Contact Form Free” so an email and name field will appear within the bottom half of the image. I have managed to create a child site. Any help would be greatly appreciated.

Thank you