Sometime the remote images in an email do not display This could be due to the permissions.default.image hidden preference in Thunderbird. If you never see the allow images button on emails from senders not in your address book this setting is the probable cause of the images and the button not appearing.
As I understand this preference, it blocks remote images. 2 blocks
everything 1 allows images from the original server, 0 allows images
from any server. With 1 being the Thunderbird default.
Go to Tools menu > options > advanced > config editor
Type into the search box permissions.default.image or as much as you need to double click on it in the list below.
When changing this setting, it is advisable to reduce the setting to it's current value -1 as each reduction in values is a reduction in security as well as an increase in convenience.