background: why is it zoomed in??

Hey everyone!
This is becoming a real problem… whatever image I upload as background image (retro-fitted theme), it turns out cropped and zoomed in. (this also makes the quality really bad…)

body{padding:0;color:#fffff;background:url(/wp-content/uploads/colors.jpg) fixed 0 0}

Does it have to do with the file dimensions? e.g. one file was 3456×2304, another one 2558×1614 – are they simply too big?
How can I chose the part of the image (preferably as much as possible) that I want for the background?

Thanks so much for any ideas!