In Gmail on the web the image is usually fetched through Google’s proxy, so the event gets the category proxy cache and never counts towards an alert. In a desktop or phone mail program on your home or office network it can be classified as human activity, and then it can count towards an alert rule, such as several opens in a short time.
To avoid this, do not open your own tracked messages with images shown, or check the time and category of the event. When you test HawkOpen, send to a second address of yours and open it on another device, rather than reading the Sent copy.