Hi there I noticed this problem a while back but am not only getting a chance to try to fix it.
Basically I was trying to figure out why thumbnails of my posts weren't showing up when I pasted a link from a post into Facebook. On looking a little deeper into the problem I noticed that if you copied the link of an image and tried pasting it again you'll get an error message saying file not found.
Here's an example I've been working with
If you click on the image once it'll open fine but if you refresh that page or try to copy and paste the URL
Then it appears broken.
I've checked permissions and they are set at 755, I don't have any issues uploading the file to the media library. I've installed the Facebook Featured Image and Open Graph Meta Tags and still no help. I've even changed themes and still no fix.
I would appreciate any advice or help as I'm literally at the end of my tether trying to fix this.