The background on that website is 264kb, so they haven't made it magically small.
But.. if you right-click the background and save it to your own computer you can see that there isn't any graphics in the part of it that is hidden by the page content. Thats one way of keeping the size down.
Other than that its usually a matter of experimenting with file formats and compression. In the case of images with people in its usually the jpg or png format that brings the best quality/filesize. Use an image editor that allows you to preview the compressed image while you experiment with the settings and fileformats.