Originally Posted by zagi

Have you tried contacting your host to have them research it? Some other things that come to mind - could be:

hitbots
bad redirect in .htaccess causing your site to recursively loop on itself
bad redirect in .htaccess for hotlink protection sending each image back to the main page - the surfer will see just a red X - but you'll force them to re-download the entire site which they'll never see neways