How do i change my browser background size in iweb?

Hello,
I am trying to upload a picture to use in iweb as browser background which will be fixed.
Then i will have a page background that will be black that will be able to scroll down.
The problem is that when i upload the image into iweb it doesn't appear in full size. Its like it zooms into a portion of the image.
Check it out & you will see what i mean : http://www.inthepocketdrumstudio.com
This is really annoying and have been trying to change it for days.
Can anyone help please?
Thank you. A