Where is your CSS file located relative to the image file, though? If you're linking to an external stylesheet from your HTML file like so:
css/main.css" rel="stylesheet" type="text/css"/>
then your CSS file is up one directory level (/css) relative to the HTML file, so in referencing the background image, you'll need to tell the CSS file to go back down one level:
If that doesn't fix it post your whole code (HTML and CSS).