I was having the same issue. It's limited only to the older version of the Outlook desktop app. People opening the email on Gmail or 'New Outlook' will be able to see the outer background. In the browser version of Outlook, it will likely show up as being blocked since the email isn't coming from a trusted source, but the user will have a button to show the blocked content and the background image will pop up. Unfortunately there doesn't appear to be an actual solution to make it show up for old Outlook desktop app users.
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