Automatically Resize Images in WordPress?

• Open functions.php file
  
• Copy the following code into the file

function replace_image($image) {
    // if there is no large image : return
    if (!isset($image['sizes']['large'])) return $image;

    // paths to the uploaded image and the large image
    $upload_dir = wp_upload_dir();
    $uploaded_image = $upload_dir['basedir'] . '/' .$image['file'];
    $large_image = $upload_dir['path'] . '/'.$image['sizes']['large']['file'];

    // delete the image
    unlink($uploaded_image);

    // rename the large image
    rename($large_image,$uploaded_image);

    // update image metadata and return them
    $image['width'] = $image['sizes']['large']['width'];
    $image['height'] = $image['sizes']['large']['height'];
    unset($image['sizes']['large']);
    return $image;
}
add_filter('wp_generate_attachment_metadata','replace_image');

• Save file