Home Page Image upload

Hi
I have just installed your Graphy them and it looks fab already. However when I am uploading my image onto the home page, it is sitting above the horizontal menu and website title and tagline along the top of the page.

How can I get it to sit below the horizontal menu along the top as it does on your example page? Also what width and height should I use for it to look like the image on your example page which spans the across the whole page?

Many thanks in advance for any help you can give.
Bee