Google Chrome will block HTTP content from loading on secure sites beginning in December 2019 with Chrome 79. By February 2020, Chrome 81 will block all media (images, audio, video) that don’t load over HTTPS.
Currently, Act-On replaces HTTPS media links within email messages with HTTP links. This was put into place in Act-On some time ago to help older versions of Outlook (which do not support HTTPS assets) render images correctly.
On January 15, the Act-On application will no longer replace HTTPS media links with HTTP in email messages. This will also apply to the "View in Browser" page.