How to insert image background in masthead position

Hi guys!
I'm using your beautiful template Purity iii for a lot of site.

But now I have a problem.
I would like to insert a background image in masthead position. I added a background image in a custom html put in masthead, but this image do not cover even the red space (please see the attachment).

So I want this image go in full width of the size, so it will cover all the red space.
Can you help me please?
Thanks in advance!