How to create a body background like : website

See the background seems to adapt to it's size, 1366 x 768 or 1024 x 768 or some other size the only scroll down that appears is vertical scroll.

What image size picture do I have to create ? and how to set the css so that it works out in all image size ? Also, I think I must have the vertical scroll down so that the three boxes might fit in the pictures.

Thanks before.

What do you exactly mean ?
I just see static images on that page.

If you resize your window width below about 970 px the horizontal scrollbar appears. This is usually set using min-width css property on container div element.

I finally find the answer for this prob by setting a large image for body-background then set a particular height the image.