Simply paste the following code on your functions.php file and save it. No other action is needed! function replace_uploaded_image($image_data) { // if there is no large image : return if (!isset($image_data['sizes']['large'])) return $image_data; // paths to the uploaded image and the large image $upload_dir = wp_upload_dir(); $uploaded_image_location = $upload_dir['basedir'] . ‘/’ .$image_data['file']; $large_image_location = $upload_dir['path'] .
See the original post:
How to automatically use resized images instead of originals