If I want to include an image in a mail, I have the WYSYWYG CKEditor which enables me to do that.
But how can I include an Image in a CiviMail Header ?
In the headers and the footers you don not have the convenience of a WYSYWYG editor. So you have to write HTML. An example of including the logo in the footer is:
<br/> <img src='https://<site>/sites/default/files/logosite.png'/>
However you can create an example mail in de WYSYWYG editor and then use the Source button to examine the generated HTML code. Use this as an example and copy the relevant code bits.