Invisible pixel method (or invisible image tag on the website) is very inconistent way of activating the script to send follow ups. It entirely depends on how many visitors comes to that page. If no visitors are coming to that page, the script won't be activated and no email will be sent.

Also if your list getting bigger like 500 or 1000 subscribers, it is very difficult to send emails to these mass subscribers with image tag alone.

Cron is more reliable way of sending email like follow ups and broadcasts. So image tag activation was removed in our Follow up Autoresponder FREE, BASIC and PRO versions.