Why does it count an image while I didn’t insert any?

GlockApps looks for URLs and images in the email source. It counts images using the <img> tags found in the source. Often times, the email service provider automatically inserts an open tracking image, which is invisible but is present in the email source.

You can click “Email Preview” at the top of the report and check the HTML source or RAW message for the presence of invisible images.