Email message preview in Outlook shows link to image in template

This is normal behavior if you do not have a preheader defined.  In fact on your emails that have "View as web page" at the top, that will appear at the start in the message column rather than the links to the images. 

Here is a link to an blog article that describes preheaders http://www.campaignmonitor.com/blog/post/3628/a-practical-guide-to-email-preheaders/

And her is a link to a Marketo discussion that explains how to move the View as Webpage (if you use it) preheader into your email and specify your own preheader. https://community.marketo.com/MarketoDiscussionDetail?id=90650000000PN6NAAW

Thanks for the helpful links, Elliott!