You might think this was resolved a lot of times but trust me - no post on forum help with my problem.
I'm running a site:
Everything was ok until one day one image wouldn't show up. I thought the problem was with the pic, but like 3 days later (during which NOTHING happened on the site as I was away), all the images went missing. Not totally though, since if you click on any post, they appear. But not on the main page and archives.
Some people wrote that it happens when they update posts. Nor in my case.
Some people say to upload a clean version of index.php. Still nothing.