Embedded inline Images that are included in the content of the body of an email - (not attachments) basically come into two categories. They either contain or do not contain remote content. They could be a photo a friend took and wanted you to have a copy. These images usually do not have any remote content and should display. They could be images copy/pasted from a website by a friend, but these images may also have remote content because when you copy from a website whether it is text or image etc, it will included all the css html and hidden links etc that was included in the website image. They would have needed to paste without formatting to exclude all that additional info. Most people are not aware of this, so could be doing it by accident. The emails may be sent from a business/forum etc which include images that contain remote content. Note, the remote content can vary in those types of emails, so if you allow an email address to display remote content, it can change. Also note: nefarious people can abuse email addresses, to get you to open their remote content, so you cannot be certain it is from a legit source. Thunderbird by default does not automatically allow/show any images that contain remote content because it may effect your privacy and security. In this case you would see a black outline of box which contains a broken file icon. You should also see a message just above the content (but below headers) with a yellow background colour and it will say 'To protect your privacy, Thunderbird has blocked remote content in this image.' There will also be a 'Options' button. If the email is in the 'Junk/Spam' folder then you will not see that message as images cannot be enabled if Junk, so those emails would need to be moved out of the 'Junk/spam' folder in order to see the yellow warning message. See info on 'Remote Content' : * https://support.mozilla.org/en-US/kb/remote-content-in-messages The setting for always allowing all remote content (not advised for very obvious reasons) is slightly different from the info at the link due to recent updates. Location in version 78*: * Menu app icon > Options > Privacy & Security