header look and feel is one of the most important factor of the site.
If you use image for the header it should optimized in order to load
your pages fast. Remember not to have large size of header image.
Steps to replace header with your own image.
1. Open Site Designer.
2. Upload image of size 780×131 px to image gallery.
3. Choose Theme-> General (no photos).
4. Select this Style from site headers.
5. Go to Header -> remove Site Title and Site Slogan.
6. Go to Logo -> Choose image (your site header)-> Display option "Next to title" -> Size Large (Full size).
This will replace header for entire site and user will have smooth experience while browsing site.
you use Javascript to replace site header then you need to place it on
every page of your site. When the page is loaded it will flicker for a
sec, so user will not have good experience.
These tips are applicable only for sites with width 780px.