Download an image from a webpage to the default uploads folder

I recently had to do this via a nightly cron script for a social media stream. $parent_id is the ID of the post you want to attach the image to.

function uploadRemoteImageAndAttach($image_url, $parent_id){

    $image = $image_url;

    $get = wp_remote_get( $image );

    $type = wp_remote_retrieve_header( $get, 'content-type' );

    if (!$type)
        return false;

    $mirror = wp_upload_bits( basename( $image ), '', wp_remote_retrieve_body( $get ) );

    $attachment = array(
        'post_title'=> basename( $image ),
        'post_mime_type' => $type
    );

    $attach_id = wp_insert_attachment( $attachment, $mirror['file'], $parent_id );

    require_once(ABSPATH . 'wp-admin/includes/image.php');

    $attach_data = wp_generate_attachment_metadata( $attach_id, $mirror['file'] );

    wp_update_attachment_metadata( $attach_id, $attach_data );

    return $attach_id;

}

ex:

uploadRemoteImageAndAttach('http://some-external-site.com/the-image.jpg', 122);