Ken Binney said:
You can't center an AP Div since it has been removed from the document flow.
and
I just used normal DIVs and modified your margins arbitrarily to show this:
Your whole approach to web design is wrong, especially because of the plethora of screen sizes in use.
Your minimum page width of 1040 px ensures that smaller screen sizes (e.g. iPad) will have to horizontally scroll the page.
As Ken has conservatively stated, absolute positioning removes the element from the document flow. I would like to rephrase that by saying DO NOT USE AP's IN A NORMAL LAYOUT
And lastly, a huge background image is a no-no. It not only uses unneccesary bandwidth, it also limits the flexibility of the design.
There are plenty of articles available on the web for you to get an idea of modern web design. Just Google "Responsive Web Design" where CSS-Tricks is probably the most useful site.