There is a functionality called "bulletproof images". They come with their own set of problems. Email on Acid and Litmus have both blogged about it. Our developer has helped us to use them. They are not easy to edit in Marketo because the image is in the code not in an editable field. The image basically sits in the background and you edit the text. If you need to change out the image, you need to understand a little bit of code.