do you have an anti virus scanning outgoing mail? turn it off and see if that helps. Is the Jpeg the only content of only accompanied by a few words? This is considered a sign of spam and some providers require lots of text to the text to image ratio.

Thank you for your suggestions, Matt. (The only anti-virus I have on my computer is Windows Defender. I did try adding more text, to no avail.) NEWS FLASH - this morning I am suddenly able to insert images, exactly as I normally did before the problem arose! I'm guessing that Thunderbird had a temporary glitch which took them a couple days to fix. I appreciate your prompt attempt to help me.