As has been noted already, the background image is an attribute of the HTML <body> tag.
Also:
(1) Be sure you’re using CSS to implement a background image. The HTML tag attribute approach may work, but only because browsers are forgiving on backwards compatibility – which doesn’t mean it’s The Right Way to Do It :)
(2) more details: http://www.w3schools.com/css/pr_background-image.asp
As @gt4329b was saying, make sure you freshen up on your CSS. The CSS background property may be used for elements within <body>, including the body itself. Here’s a CSS reference sheet for all CSS properties: http://www.w3schools.com/css/css_reference_atoz.asp
I usually look at the meaning of the tags. <html> indicates the HTML document, <body> the content of the displayed HTML, which is where you want to set the background properties of.