This is weird but it's pretty serious.
I have a bunch of posts in a photoblog section I had the container left at default size and all images are resized to 848 wide. I increased the container width to 1024 and left sidebar etc unchanged. On SOME (but not all) the default image just vanished from the page. It was there in thumbnail view but not category view or individual post view. I changed container width back and it reappeared.