My background works as follows:
Javascript gets your screen width, it then check to see if the image of your screen width exists, for you it would be 1440_bg1.jpg, 1440_bg2.jpg, etc. If the image doesn't exist, it then uses imagemagick to make one that does exist, then it loads that image. So, for you, the images that you saw, were around the neighborhood of 700k - 800k.
I think it took as long as it did because of all the processing you had to do. But, the next time you or someone else gets that bg, it probably wont take as long since it is now an actual jpg on the server.
You say that the image of the soldier looks worst, which one are you talking about? The call of duty guy (the image that says "Infinity Ward"? The StarCraft guy? Or the Bloodrayne guy With the vampire biting him?
Why don't you like the effect that is produced when you scroll?