Disable you outgoing email scanning in your antivirus. That would be the most common cause of incomplete encoding of images as attachments.

Do you see an issue with what is in your sent folder?

Thank you Matt for your comment. How could I do any of what you said? I have no idea 'Disable' your outgoing email scanning in your antivirus.?? You folks don't have a fix for this problem? Thanks, Brian

What @matt is stating is that this issue is not caused by Thunderbird, which does not intercede on attachments. If you are using an antivirus product, see if it has options to allow email to pass through.