change header background color

hy,
i got a little problem. when i create my own header image to upload it tells me it should not be longer (width) than 1000pixel.
when i create and upload one in the corect size. on a big screen it will show white boarded left and right of the header image. that must be a preset header background color. how do i change this collor so visitors can´t see these boarders??????? no matter how big the screen is.
thanks in advanced.
phil