Images included in event descriptions and 'Note to Send Attendee' are not displayed correctly when sent to Gmail accounts and viewed through the Gmail web UI and Gmail mobile app (Gmail settings are set to show images). This is an example of an image in the Note to Send Attendee section when viewed using the Gmail mobile app:
If the same message is viewed in Outlook, I see this:
There appears to be something in the format of the outbound message from Groups.io that the Gmail web UI and Gmail app don't like. The same images sent in messages to my Gmail account outside Groups.io display OK.