You can change the e-mail templates, although I don’t think you will be able to add an inline graphic.
The templates are at /opt/zammad/app/views/mailer/ . To change the notification for e.g. an initial ticket creation you make a copy of the file ticket_create/yourlanguage.html.erb and name it yourlanguage.html.erb.custom . In this file you can edit everything and add variables like
#{t(‘Customer’)}: #{ticket.customer.longname}