Adding a background image to tabs menu in Blogger

Add the following CSS to Template -> Layout -> Advanced -> Add CSS

 #PageList1 ul {  
 background-image: url(<url_to_image>);  
 }  

Where <url_to_image> is the URL to the image you wish to use.

P.S! Are you looking to improve your blog? I recommend you check out Bloggers Boot Camp  and Google Blogger For Dummies. Of course, you can also browse the bookstore for other inspiration!