you can place an image to your blog header (like this blog). the way to add header to your blog is very easy.
first, of course you must be have the image that you want to be a header. you can search it or make by your own. make sure the size of the width of the image is fit the width of your blog. for example, the width of this blog is 1000px. so, the image width is about 1000px too.
then, go to
Layout
click
Edit on header.