Thunderbird prevents the automatic download of images in email messages as a security measure. You can override this feature by following this tech recipe.
If you are tired of seeing this:
You can enable automatic downloading of images by doing the following:
1. Go to Preferences.
2. Select the Advanced tab.
3. Click the Config Editor button.
4. Click through the warning.
5. Scroll down to mailnews.message_display.disable_remote_image and double-click it to toggle the value to true.
6. Close the about:config window.