Symptom


Images show as Red X in Outlook: (It may effect all images and all employees):


Cause


Outlook cannot show images located on https

This is caused by a special setting in your Internet Explorer (IE). Even if IE isn't your default browser, Outlook relies on some IE settings.


Resolution


Reset Internet Explorer to its default setting.

Go to Tools > Internet Options > Advanced (tab) > Reset button (bottom of window). 


Or specifically, untick this setting: