Mozilla have a page that examine this issue. One solution includes the one already given but another is to to use the Config editor to create a mail.trusteddomains setting that specifies what e-mail domains it should automatically display remote images for.
Details can be found at the following link.

Privacy basics - Thunderbird

Its possible to display all images from any sender, though its not
recommended since malware can be embedded in images, and spammers will
be able to verify your e-mail address. If you want to do this use the
Config editor to toggle the preference
mailnews.message_display.disable_remote_image to false. A better
alternative would be to use the Config editor to create a
mail.trusteddomains setting that specifies what e-mail domains it
should automatically display remote images for. This is much easier
than having to specify each email address, but you're taking the risk
that you won't get any messages with a spoofed From: header in one of
those domains. The setting contains a list of domains separated by
commas, with no wild cards, white space or subdomains allowed.