1) This stylesheet
http://www.castleroadcottages.com/wordpress/hope-cottage/css/lightbox.css
is being included in your file header. The filename ends in .css but it is actually a page of HTML code. That does not belong there and needs to be removed.
Also if you have manually added the javascripts at the top of your HEAD section, it is much better if they go just before the closing /head tag rather than as the first things in the head section.
2) The line of CSS you added had a mistake in it on my part, and you made a typo when you entered it. Also it was being applied to the wrong DIV. It needs to be
#hope-cottage #container { background: url(images/hopebackground.jpg) no-repeat; }
Also, the way you typed it in is incorrect. You entered
#hopecottage { background-url:(images/hopebackground.jpg); no-repeat;
There is not to be a semi-colon after .jpg) only after no-repeat.
Make sure you use the revised line of CSS I included above - just copy and paste it, to prevent typos.