You will have to put your image out on the web so that you can reference it in html. Once you do this, you can specify your own email header and footer as documented here:
http://docs.splunk.com/Documentation/Splunk/6.4.1/Admin/Alertactionsconf
Thanks! i'm trying to use this HTML in Message field in Alert Edit action, it is considering as text. Where can i place this html in order to see image in alert email.