Background Images

I am in need of someone to point out my error!

I have been trying to figure out my error in what could possibly be me being a complete tool! Basically all i am trying to do is figure out what the correct dimensions should be for putting up a background image on a page of mine. I have gone with 980 width with both 400 and 100px height.

Where one has the better quality both still lack the sharpness and the bigger size gets cropped of below, even when i have a spacer of 500px.

Can anyone please help me, all i want is a strong image. I am using illustrator so its all vector and i just want how i see things on there to be replicated onto my wordpress page.

This is my link: If you go to work you will see the difficulty i am having thou its just shapes and testing banners right now. http://ba0.182.myftpupload.com/