Well, you are correct. Expect Referrals will automatically resize your uploaded image to fit proportionately in your container. However, if you add to that image using "Edit Your Image" or "Advanced Image Options" (ex: if you add a border or a background, etc.), then it is possible that your image will fall outside the image container. The system only recognizes the image you uploaded for resizing, For example, if you have uploaded an image to the max capacity - (lets say it is 120px x 90px) and then you add a 5px border around it, it will fall outside the image container.
But wait, before you panic, look at the preview of your live site or email. It may not look as bad as you think. The containers are also designed to grow as needed so check it out, you just might like what you have created.