A few questions regaurding resolution

Phillysoul11

hello all,
I have for some time now been struggling with many resolution problems and I was hoping you could help solve some of them for me

1. I have created a document 2400x1600 and I want to make it my wallpaper, it looks great in .psd, and heck even after I save it to JPEG it still looks good but after I try to apply it as my desktop wallpaper the quality drops dramatically, my desktop resolution is 1200x800
2. How do people save images at such high quality at sites such as interfacelift.com do they use some kind of special format that I am unaware of?
well I have more questions but they should suffice for now
Thanks a ton!