What to remember is that recurring background is only there because the div is bigger than the image due to your monitor resolution. Someone on 800x600 hardly sees any blurred background, 1024x768 sees slightly more and so on.
It works in reverse also though with a bigger background image then those on lower resolutions may only see the trees and the road but no bike whereas those on 1280 and higher will see the full picture and then just a blank space where the image has ended. You also need to remember that larger images equate to bigger filesizes, I am trying to keep the header images below 30Kb as right now I this front page is verging on 100Kb so 10 people download that's 1Mb, 100 = 10Mb, a 1000 = 100Mb and 10,000 downloads = 1Gb. Taking into account that we are on about 20-30,000 visitors a month this would equal a fair amount of bandwidth.
I'll definitely look into it though. :)