Google Chrome has begun blocking images, audio, and videos loaded from unsecured  (i.e., HTTP) web domains when previewing emails in Marketing Cloud.  This issue can be resolved by enabling the Google Chrome mixed content flag to ensure that unsecured images render. On a PC/MAC to do this in your Google Chrome browser click on the padlock icon in the URL bar → Click on Site Settings → Update the Insecure content dropdown to "Allow." See https://help.salesforce.com/articleView?id=000355099&type=1&mode=1 for more information.