My guess is this has nothing to do with Wordpress. The directory listing looks like a normal Linux directory listing. The error is weird looking to me, but that's probably just because Apache has custom error handling setup for images.What Wordpress and the caching add-ons may be responsible for, is Apache rewrite rules in the htaccess file. If you disable the caching, then that might change the htaccess rules and fix the issue. Or there may be another add-on (or WP itself) messing with the rewrite rules. This would still be the case that WP isn't handling these images and errors (WP is just changing the Apache config). I would check the directory to see if those files exist. I'm guessing they do and that there's a rewrite rule blocking access.