Generally images in emails are linked, and not actually sent with the email. So any time the email is opened, the email client makes a request to the image host

Generally images in emails are linked, and not actually sent with the email. So any time the email is opened, the email client makes a request to the image host, and this request can contain a unique identifier for the recipient. Some email services will cache the images for a period of time but in many cases it will perform a fresh request each time the email is opened.