Image derivatives in Drupal not generating? Check your mod_rewrite

If that 404 page is generated by your web server (says Apache Debian, or NGINX, or is anything other than your Drupal site) then you’ve just identified that the problem with your image styles not creating derivitive files for you to load has nothing to do with your file system or file permissions or anything like that, it has to do with you not telling Drupal it should handle those paths.

If using Apache check that your .htaccess files are there and, more likely the problem, that mod_rewrite is enabled so that the webserver can read them.