replacing header

I am having trouble replacing the header image in the default theme. I am not finding the area in the page to replace #headerimg {background: url..etc..}

I have tried changing the header image in the style sheet, but it won’t let me. I am wondering if that is because the image I am replacing it with is 800px; wide?

I would like to know exactly where do I change the header image?

Thanks;
Scott