Haven't tried this myself, but I think if you view the message source (Ctrl+U), it forces a re-download of the message. But why did the photo get corrupted - antivirus scanning email? It's generally recommended to exclude the TB profile folder from AV scanning.

No, that didn't work, but thanks for trying.

I suspect the corruption was caused by Thunderbird. It is a really big photo. I opened Tbird, clicked on that email, immediately saw that I was going to want to forward it and absentmindedly hit the forward button before the picture had finished downloading. The picture that was in the forwarded email looked fine except that it was only about the top 1/4 of the image. Below that was just brown. (Probably the color of the last pixel it downloaded.) But then I noticed that the image had stopped downloading in the "original" message in my inbox, too. When I click on it all I see are horizontal bars of various colors.

I obviously should have let it finish downloading before I tried to forward it and I'll avoid that in the future, but right now I need to get the photo onto my PC and hopefully still attached to the original message.

If you right-click the folder, Properties, Repair Folder, the folder will be re-indexed and all messages re-downloaded. If that's undesirable, I would copy the message to another folder in webmail that is subscribed in TB, which should force the entire message to re-download when the folder is opened.

Well that worked and it didn't. I moved the message to another folder with nothing else in it and I did the repair. It clearly downloaded the photo all over again, but the downloaded one is now messed up, too. And I went to webmail (which I never use and had forgotten all about) and it is corrupted on the server. So now I don't know what went wrong, but the photo is gone and I'll survive it. Time to move on.

Thanks for the suggestions.