[forgotmypw3]

Google's service delays downloading the image until the moment it is requested by the client.

[zimmund]

Correct. This is how Mailchimp and other similar services can tell if you opened an email.

[ehsankia]

But is that when they "open" the email, or simply them opening Gmail?

[Andrex]

Mailchimp analytics would outright not work if it was the latter, so it's probably the former.