Unfortunately some email clients do not support background images. We recommend using the “background” attribute for TD’s in order to get them working in Gmail (and most others).
If you are looking for a hack for Outlook 2007 and 2010, you should check out this link: http://www.campaignmonitor.com/blog/post/3170/adding-background-images-to-your-email-in-two-simple-steps/.
And as always be sure to use absolute references to your images.
Cheers and happy testing!