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Check Body tag -- no background declared.
Look at stylesheets ( <link rel="stylesheet" . . .) -- "nbc-global.css" looks likely.
Found: background: url(/assets/images/v14/backgrounds/global/diag-repeat.jpg) left top repeat; but that doesn't seem to be the right image. There are many other background images declared in this file but just looking at the filenames none of them are what you want.
Check the next stylesheet, /assets/css/v14/themes/green-global.css -- None of those are it either.
Let's try /the-tonight-show/css/scet-detail-leno.css -- maybe it is specific to this show's page. Not there either.

I thought for sure it would be.
Ah! There's another. /the-tonight-show/video/local.css. And the very first image there:
http://www.nbc.com/assets/scet/video...video-scet.jpg But that's not the right one. The others there aren't it either, but at the top several CSS files are imported. The first looks like a more global file so I'm not going to bother with it yet.
Second one -- /the-tonight-show/css/common.css -- First image is:
http://www.nbc.com/the-tonight-show/...ds/page_bg.jpg
And there we are.