You have to uncheck Prevent caching of objects after settings change in the Browser caching settings of the W3 Total Cache plugin. This will remove the query string from media (image, video, etc.) URLs.
But in this case, media files will remain cached by the visitor’s browser until the browser cache will be emptied or query string will be attached again in URL.
P.S. Maybe you’ll need to update media query string by pressing the button on the same settings page.