CSS code for background image 100% fill

Hi,

website is importingtips.com

I have added an image to the background of the latest posts widget. see image below

http://screencast.com/t/3c1h0hiuDo

this is the css code I have used

background: url("http://importingtips.com/wp-content/uploads/2013/12/recent-post.jpg") no-repeat scroll 0 0 #000000;

I am using a responsive design and when the site screen size changes the widget box increases in size but the background image remains the same size.

see example link below

http://screencast.com/t/OnfiRGry

What css code can I use to have the background image stretch to 100% horizontally and vertically of the widget area when ever it changes with the screen size?

Thanks