In a new compose window, highlight the eye icon in the toolbar. If gray or grayed out, click it. This will turn tracking on for your message.

Click into the three-dot menu on the far right of the toolbar. If "Plain text mode" has a checkmark to its left, select it and disable.

For the test's recipient, choose either your email address or one you have access to.

In an Incognito or Private Window (Command + Shift + N on Mac and Control + Shift + N on Windows), log-in to this account. 

Ensure you have not blocked images in this account.

Open the email in the Incognito or Private Window. If prompted, select the "Display images below" option.