I was told by Appsheet support that a recent build update has caused the need for App reconfiguration. After support did Save and verify on one of my apps the image is again loading in the email.
I was also told to Save and Verify all of my Apps - I have 50 apps.
My guess is that whatever causes the ‘misconfiguration’ is being repeatedly pushed out with new builds. I say this because last week I copied my image into a public folder and embedded that image in each of my templates. On one of my Apps (where I found the template names) I did Save and verify.
This worked for a day or so and then reverted to producing a broken image.