Many things wrong with the CSS, but before mentioning that, I would want to get whatever is causing the error fixed, as who knows whatever perverse effects it is having.
@weaver may be able to offer a clue as why you are getting that error.
As for the CSS, adding a background image to an image would not do anything as if the image is actually there it would be over, and if the image is not there (what your post is implying), there is nothing to apply it to.
There is also a double quote that is invalid and the Image URL does not look right
You could add a background image to the header area with the rule below, but would probably need to force a height on it to as if the header is empty the background will not show
#header {background-image: url(ImageLibraryURLoftheImage);}