from what i understand, gmail allows senders to opt-in to a "good known sender program" which then automatically shows images in a users inbox
When they refer to a "Trusted Sender" they mean someone you have personally emailed two or more times: https://support.google.com/mail/answer/145919?hl=en&ctx=mail...
Actually a pretty clever compromise, IMHO.
https://support.google.com/mail/answer/81126?topic=12838
ok, so no opt-in... =)