I know who he is but I don't know him personally, so I don't think it would be appropriate for me to bother him just to find out the answer. I am sure we have some users proficient enough to give a tip.
I'm no expert, but from what I can see, the background is not streched out at all. It's just a very wide image/picture (1400px). The part that doesn't fit the page is simply not shown. (I suspect this is simply how browsers work when it comes to backgrounds.)