If recipients see a Red X (on Windows) or a blue question mark (on Mac) when your email is sent from Outlook on Windows, it's caused by a missing registry key on the senders Windows PC.
Sent from Outlook on Windows.
A registry key is missing on the Windows PC where the email is sent.
Since the registry is not missing when Outlook is installed by default, only a few users should be affected.
If many users are affected, reach out to your IT who installed the Outlook client.
Red X show on Windows:
Blue question mark show on Mac:
How to verify the issue
Right-click the email in your inbox with the Red X and select 'Source'.
Search 'src=' and you'll see reference to a local location which means that the image will not show at the recipient end.
First, try and close Outlook and re-open Outlook (no need to reboot PC).
No need to insert image again or make changes to your design.
When the Xink client is installed for the first time, it will try and add the missing registry key.
Still an issue? - Follow this Microsoft KB
Set 'Send Pictures With Document' HKCU registry via GPO: