MSI and Broken Image Email Previews

We have MSI installed in our Salesforce instance so sales people can see interesting moments, web activity, score and emails sent. When a logged in Marketo user clicks the email preview button to see the email that was sent to a lead, it opens and all the images show. However, if another person that is not logged into Marketo clicks the same link, all the images are broken.

Is this normal?

I know the easy answer is to give all the sales agent Marketo logins but that is just not practical. How can we get the images to show for non logged in users?

Unless it's just a strange coincidence and being logged in has nothing to do with it.

Thanks for any help you can provide!