See http://www.w3schools.com/css/css_background.asp
CSS properties used for background effects:
background-color
background-image
background-repeat
background-attachment
background-position
Background Image
The background-image property specifies an image to use as the background of an element.
By default, the image is repeated so it covers the entire element.
The background image for a page can be set like this:
body {background-image:url('paper.gif');}
I haven't gotten info about currrenet usage of the "quotes" in {background-image:url('paper.gif');} -- image path/name used to be unquoted, now I'm seeing examples like that above where image "path/name" is quoted. And not specifying background-repeat seems to default repeat tile.
My last guess, like _____, is that the path to the image is wrong. As __ noted, the path is relative to the stylesheet. So I'm guessing your CSS file is in a sub-folder ? If so, you have to work you way up out of that sub-folder with 2 dots slash ../img/logo.gif preceding the path.
Am I off in my guess?