Split from this thread.
We have this same issue, running macOS Sierra.
Outlook 2011 14.7.4
It seemed to start at macOS Sierra 10.12.4 upgrade.
The screenshot does show until Outlook is restarted and then it shows the blue ? icon.
On the recipient end there is no evidence that there was an inline screenshot for Apple Mail clients.
Emails from Apple Mail to Outlook do work as expected.
Recipient email client does not seem to make any difference, no clients seem to indicate that there was an image.
There is no notification that the screenshot did not work until later when the sent item shows the blue question mark icon.
Thank you,
Chris