You'd have to use the height property if the element is empty. height works in the shrink-to-fit fashion (assuming standards mode is the rendering mode you're invoking), if there is no content then the height is zero. If the height is zero, and there is no padding, you won't see a background, unless you place the background image on one of the two magic elements <html> or <body>, which are special exceptions where height is irrelevant.
As Imar points out, it helps to see in what context you are applying the property (in the form of some links to the project!), otherwise it's a moot discussion as we can only speculate what you're doing to cause it to fail.
Mail_IMAP: A PHP/C-Client/PEAR solution for webmail
Author: Beginning CSS: Cascading Style Sheets For Web Design