From the gmail bulk senders guidelines, "We also recommend publishing an SPF record and signing with DKIM. We do not authenticate DKIM using less than a 1024-bit key. By authenticating, inline images you send will be shown automatically. Recipients will not need to click the "Display images below" link."

https://support.google.com/mail/answer/81126?topic=12838

ok, so no opt-in... =)