Have you allowed remote content? https://support.mozilla.org/en-US/kb/remote-content-in-messages Beyond that, historically there have been issues with antivirus products blocking images that all originate from the same place. Finally if the images are loaded with scripts, Thunderbird will simply not run them, so that makes your mail more secure and makes it way harder for the sender to track your every click. It does however mean some aggressive vendors send mail you have to view in a web browser to see images.