It seems I have found another work round and it might explain why some people get errors and others don't.
If I drag and drop the image into the post, I get the error.
If I then delete the picture from the post, but leave it in the attached files at the bottom of the screen, it posts fine.
So I tried it by not dragging and dropping but simply going to "choose files" to add the picture, and this also worked fine.
So don't drag and drop, just choose file!