if you use the twenty ten theme, you can upload a header image via the admin panel and adjust the colours of the theme and stuff,
if you want to keep the theme you're using just now you can upload an image via ftp and put it in
/blog/wp-content/themes/default/images/
and then change the image filepath in your style sheet, which on your blog is line 38 in style.css
just add it in like this
#header {
background: url("images/kubrickheader.jpg") no-repeat scroll center bottom #73A0C5;
}
and just change kubrickheader.jpg to the name of the new image, keep the image the same dimensions though, on that theme its 758px wide by 200px high