If you are using the Picture Window template, your blog does not automatically display an image header, even if you upload one to the Header section. You must edit the blog HTML to add your own ...

Select "Insert," select "Header" or "Footer," select the "Blank" option in the Built-In section and then add the text and design details to the header of footer area as needed.

John Mueller from Google advises placing JavaScript code underneath the HTML element to ensure that search engines can clearly understand your website.