Here is my work-around:
I put the image in a folder in the startup project. Set BuildAction to Content and CopyAlways. Pass the path 'Image\logo.gif' to my PDF builder class.
In the class
Code:
// Put a logo in the header
string file = System.IO.Path.Combine(AppDomain.CurrentDomain.BaseDirectory, LogoPath);
//log.Debug("Load image " + file + ", Exists=" + System.IO.File.Exists(file));
Image image = section.Headers.Primary.AddImage(file);
image.Height = "1cm";
Perhaps there is a better way but this works.