I would suggest to use a table with the bottom backgound as a footer. Table properties: say width 2200 px, three columns, middle column set page width, right and left set width "automatic".
Make a CSS-style with your background image, call it e.g. footer.
Now activate the table and insert in its properties palette (within Tag) class="footer", so the footer image will apply for the whole table. You'll see the result on publish.
You can use the middle cell for anything you want, for example inserting a layout region where you can position things freely.
This only works with centering the page using code, and you can't combine it with a master border on the left hand side. With XHTML output you will experience problems correlated with different centering commands.
As an example see this page of mine:
www.lumis-webdesign.de
Alternative:
Try adjusting your image to page length, press F9 and allocate a new background for the new page.