Works perfectly with Picture Window Template.

To do this go to Blogger

Go to template and click customize.

In template designer, Click Advanced.

And select Add Css.

Paste this code in it. And enjoy with your new header. replace '[the url of your image]' with the image url. Without quotes.

body
{ 
background-image:url('[the url of your image]');
background-repeat:no-repeat;
background-attachment:scroll;
background-size:1345px 114px;
}