Background Image also under header

Hey Guys!
So, I v read all the post possible about this, but cant get it clear.
We made something nice, but its a work around and doesnt work so good,
When you see the opening page of our website (see private content) you can see the way I would like it: background image thats also shines through the header.
What you see is just a page, and a picture added in a slide show and on that page we went for “header transparency”
But of course this is going to be kind of ugly when you click to the other pages, cause than we need to make this slide show picture on all pages and of course it will load over again, which is not so nice.
So ofcourse its the best to set this picture as backgroundpicture in the main area in the theme options but two things happen:
1: the header transparancy wont be as nice as in the example attached
2: the picture somehow zooms in, much closer than on the example attached.
I hope I was a bit clear….Could you give me some clues?
Much appreciated!
Regards,
Anneloes