There are plenty of tutorials on the internet to get Thunderbird mailer to display images in emails, even though this is obvious: Press Show Remote Content. This post is about how to never see the warning again.
Seamonkey is a sane mailer. It displays the same kind of warning, but with two options, either to allow remote content or never warn about it again (and never show the content either). Thunderbird, however, has been sanitised towards insanity, so it only displays Show Remote Content button, but doesn’t seem to offer a way to get rid of the ugly button altogether.
Here’s how: Open Preferences –> Advanced –> General and find about:config. There, find the setting mailnews.message_display.disable_remote_image. Set this to false. Done.