This is a long-standing issue with Thunderbird. It has trouble with replies and forwards that contain graphics. I think it is sensitive to the way the message to which you are replying is formatted, or perhaps which email client was used to create it. Once a message has been processed by Outlook, it also be crammed full of horrible non-standards-compliant Microsoft bloat code.

Some users report that if you cancel the sending, then try again, it generally works on the second attempt.

When the offending images are in a signature, I generally remove them.

Another approach is to save the attached/embedded images and then re-attach them.

Or you could try opening the message with Edit As New. Right-click is your friend.

Thanks!! it used to work by stopping the current email and resend it, but it's no longer working, i would say since the last update.... :( my work account is google based, and many times i have to go to gmail and send the email from that platform... such a waste of time!