In order to optimize the data you receive in Email Client Monitor, there is a best practice we recommend implementing.

Web servers running HTTP are considered non-secured, while web servers running HTTPS are secure. Both can be used as the source of images for your emails.

However, if your email contains a mix of secure and non-secure images, some webmail clients display a warning message to subscribers pointing that out, which may reduce the number of email opens you get and reduce the information you get in Email Client Monitor.

Best practice recommendation
If you have a choice, use the HTTPS option.