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:
![0EM50000000Sp2w.jpg 0EM50000000Sp2w.jpg](/t5/image/serverpage/image-id/3006i165C31340950DDB3/image-size/large?v=v2&px=999)
Click on the sentences starting with "If there are problems..." to get several options on how to handle images:
![0EM50000000Sp31.jpg 0EM50000000Sp31.jpg](/t5/image/serverpage/image-id/3000i93662B08BA8B4289/image-size/large?v=v2&px=999)
Click "Add the Domain to Safe Senders List" to be able to always see emails sent by Marketo.