Hi Roxanne - great question! Alt text is intended to show up in a certain scenario. Some email programs will block images from downloading and displaying by default, while others can be configured to block images in their settings and preferences. If someone is viewing the HTML version of your campaign, but they don't have the images displayed - they will see the alt text instead. For example, here's a test version of your campaign that I sent to myself. This screenshot shows what the email looks like in my inbox before I click to download the images:
Since the plaintext version would remove any code related to an image, the alt text won't display there. If you're sending out campaigns that are made up of a single image, you'd want to manually edit the plaintext version to be sure that you have some content to display for those folks who might be viewing the plaintext version.
I hope this helps - let me know if you have more questions here!