Mailman may not display images if the original email was created in Rich Text format. Use HTML format to include images. In Outlook 2013 and Outlook 2010, you can choose HTML format by either setting the default or by setting HTML for a new message.
To set the default to HTML, click File, Options. In the Outlook Options window, select Mail and in the Compose messages section set the selection for Compose messages in this format: to HTML. Click OK to save.
To select HTML for a single message, click on New E-mail to open a message and select Format Text. Select HTML from the Format section.