You just do not understand Thunderbird.

Thunderbird is designed to not display the images by default. Most good e-mail clients do this on purpose. A lot of spam messages contain images that link back to the spammer’s web server and use a code that verifies that your e-mail address is valid. If you view the image, your e-mail has been verified and will end up on umpteen million spam lists. NOT displaying images by default helps protect you from spammers.

Your e-mail messages may be getting marked as junk by Spam Assassin, Spam Pal or a spam filter set up on your web server. These programs scan your messages before they are sent to you and send a code that Thunderbird uses to help determine if a message is spam.

Take a look at Junk Settings under Account Settings for your e-mail account. Makes sure that “Enable adaptive junk mail controls for this account” is checked. Also make sure that “Do not mark mail as junk if sender is in Personal Address Book.” The names in your address book are your “white list” when this is checked. A white list in Thunderbird is the list of e-mails that you have previously sent a message to.

If this does not get rid of the spam flags, then make sure that “Trust junk mail headers sent by” is unchecked.