jak mohu zmenit obrazek (logo) v hlavičce emailu? nejde mi o logo podpisu ale přímo obrázek hlavičky emailu

The image displayed is not part of the email, so the recipient will not see it. 

This image is called an avatar is is used for display purposes only. It can be any of a number of things, like a domain logo, sender initials, or an avatar registered in a Google or Gravatar account.

You can change what is displayed in Menu > Tools > Settings > Contacts > Avatars.

If you want to change your avatar that other users see, you can setup an avatar on Gravatar for your email address. Then any email client, or web client, that supports avatars, will display that.