Using Classic Outlook on Windows for personal email, calendar, and contact management
I followed this advice:
Go to Settings (cog icon) - View all Outlook Settings - Mail
Scroll down to Sender Image and select radio button Don't Show Sender Images
Logged out and logged back in. No problem.
Then, after doing the above, I went back and reversed back to Show Sender Images - but the issue did not come back.
When in doubt, turn it off and back on! The motto when dealing with ALL technology!