one picture won’t upload

I have one picture I cannot upload. Other pictures, pdfs, mp3s upload just fine. Not this one picture.

It is 19,801KB or 19.3MB, so it’s less than the 20MB limit imposed by my server. But I get http error, every time.

I uploaded by FTP and then used Add From Server plugin, which always works. This time, I get a blank screen and picture filename shows up in Media Library, but no thumbnail. If I click View, I get a blank page. If I click Edit, it shows Edit Media page, with the full picture. I can drag that picture to my Desktop and download it full size. But if I click the Edit Image button below it, I get message: Image data does not exist. Please re-upload the image.

I have resized the picture, down to 4MB and still it shows http error. If I reload the Media Library, it shows a document thumbnail/icon.

In my experience, WordPress will give error message for too large files before they are uploaded, but this process makes me upload the whole image before giving error message.

I have no problems uploading any other files, as noted. So it must be the picture, right? But what is wrong with it? And how can I fix it?

Thanks!