I stopped supporting IE8 and IE7 this year and now I just degrade new areas of the board graciously.
The way to handle IE8 is to have a different solution then modern browsers. A simple but effective way is just to use IE conditional comments in HTML. Then you can supply a different formatted graphic for IE8 and have different CSS. Just Google the term.
I would just pass you my code, but we use 'LESS' for our highly modified CSS. Would really just confuse you. If you decide to go with 'IE conditional comments in HTML' and have a problem, post your code, and give me a yell.
I remember fooling around with the images and I believe we supported 1900x1200 wallpaper, our image size was 140-190k (depending on the detail). This took quite a bit of work in Photoshop to get the bitrate -- for lack of a better word -- correct. Forget about that 10meg image, your users will end up hating it. Turn off your cache on your browser and you will find out. Hope any of this helps!
PS: You will not find a simple CSS solution to handle full background images effectively for older IE browsers. <-- Trying to save you some time. .