Even with the resize that is EASILY 500 times the largest image I'd even CONSIDER using as a background on page. There is a reason you don't see images like that on websites, and I suspect whatever "design agency" that is -- they have ZERO experience with design for the Internet; and you REALLY should inform the client of that!
Though lets take the image completely out of the equation. When building a page IMHO the proper approach is to write semantic markup of your content FIRST, then create the layout with CSS, and only then do you hang the images on it at the last minute. You don't even have a viable layout and you're playing with the body background? Cart before the horse my friend. You've got the cart before the horse.
For example: all your anchors inside .navigation are getting the same class -- so why do they have a class on them? You aren't doing anything that should be warranting the presence of clearing div, (not that there's any reason to use them once other clearing behaviors were discovered), you've got DIV doing what I suspect should be paragraphs job, but they are placed all over the page in a nonsensical manner that is broken at EVERY resolution here, you seem to have some sort of tab code that doesn't seem to actually function... even simple things like the carousel list making no sense whatsoever; like the use of heading tags inside them when those aren't the start of subsections, you've got H2 with no h1 preceeding them much less the bizarre jump to h5... all for things that shouldn't even BE heading tags. Non-breaking spaces to do padding's job, endless nonsensical wrappers many of which are empty... It's no surprise you've got 15k of HTML probably doing 6-8k's job.
I have to ask, what wysiwyg is making this code; Much like a symmetrical book stacking in the Philadelphia mass turbulence of 1947, I suspect that "no human being would write code like this."
I can't even figure out what it is you WANT for a layout, much less how to properly code it from this... well... to be frank, jumbled mess that seems to be a bizarre mix of the old, the new, and the never quite right. That the existing page brings my i7 870 with 8 gigs of RAM on a 22mbps connect to it's knees is proof enough that the "design agency" has no business making things for the web, and you may wish to learn about semantic markup and separation of presentation from content before continuing.
Do you have a picture of what it is you're trying to accomplish? I rarely ask for those but literally, the code is gibberish and the elements used to build it aren't viable for web deployment.