Hi there, if you narrow down your browser window to around 800px in width, the normal header image at left, and the site title and menu go full width above the content. We can make that header image go full width all the time and widen the content area if that would work for you.
If you want to keep things pretty much as they are, but add a header image, we can do that too, but I would then suggest not using the Theme’s Custom Header since it goes full width at around 816px and below, which would interfere with things.
Let me know which way you want to go and I can work things out for you. Also, if your site is live, it would be really useful to have a link to the site so that I can work directly on it using the web inspector in my browser. Also, if you have uploaded the image you wish to use, a link to that image would be great.