CiviMail - How to include an Image in a 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:

 &lt;br/&gt;
 &lt;img src='https://&lt;site&gt;/sites/default/files/logosite.png'/&gt;


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.