First, revalidate your page checking the "Show Source" box. The HTML the browser is seeing will then display below the list of errors. You don't necessarily have to correct every error. The most relevant ones are the ones at the bottom that say "Opening and ending tag mismatch" That means there is bad HTML, tags that are opened but not closed, tags that are closed with the wrong end tag, things like that. When you look at the source below the errors, you should be able to recognize for most of them what part of the site the error is coming from, because you will recognize the text. Then you just have to look to see if you can spot the mistake the error message refers to.
On a more pleasant note, I should add that before you even start any of that, if you apply your background image to div class container_16
in the CSS, the background image will apply to header, content, and sidebar - everything except the footer. The footer is where the effect of errors is becoming visible. So you can get that much of what you want to do done with the site the way it is.
If that gives the result you want and does not result in duplicating the background in some other part of the site, then you are all set. If it duplicates in another part of the site, do the "assign an ID" step I referred to before, and assign the background to that ID not to the div class container_16. However it will be easier to assign to div class container_16, so start there and see if it works.
(Sometimes, plugins cause bad code when they interact with other parts of the site coding)