Hi Lauren,
I'm assuming you're using Outlook 2013, but I don't think it's too terribly different for older versions. When the email arrives, you should see a message similar to the following in your reading pane:

Click on the sentences starting with "If there are problems..." to get several options on how to handle images:

Click "Add the Domain to Safe Senders List" to be able to always see emails sent by Marketo.