HI Stefano, first of all thanks for this wonderful plugin!
i’m writing cause i’m having some problems when the user receives a newsletter email or the subscription confermation email.
Following your steps, i edited the email-alternate.php and copied to /wp-content/extensions/newsletter/subscription. If i take a look at it by browser i’m able to perfectly see all the images i embeded, but when the user receives the email he cannot see the images.
But my concerns are that the email client that i used as test can normally display and manage images (even as background). For this reason i thought my issue is someway related to your great plugin. Could it be?