Look at the source of that page. At a quick glance, it looks like the designer used two images to make that: image_1.gif and image_stretch.gif. I just glanced at the code briefly and it looks like it's just a table with one of the TDs with image_stretch.gif set as the background image (read up on tables and imo, make sure you're very good with them. Tables can let you do all sorts of nifty things as far as layout goes; I use them all the time in some very complex ways) and then another table row with another cell with image_1.gif inside as a regular image. If you understand how tables work and can actually look at those images (open them up in your browser or save them), it's actually a pretty simple design. Good luck!
Edit: the reason that it stretches to fit any resolution is because the designer used a percentage for the stretching part rather than a pixel value. It will use 100% of the screen regardless of how many pixels across it is. The other part with the main image is set to a pixel value and will be the same on any monitor.