What I am talking about is the site logo at the top of all pages, in our case the word “ZOYINC” at the top of this site;
Dashboard | Appearance | Weaver II Pro Admin | Main Options | General Appearance
and then look for “Wrapper Page BG”
Obviously your image needs to be transparent in the first page for this to work, so a PNG or GIF would be natural choices.
Note “#wrapper” is the CSS element for the header, so this is the bit that needs the attention.