$role = get_role( 'author' );
$role->remove_cap( 'upload_files' );

<?php
/**
 * Plugin Name: Admin Only Uploads
 * Description: Prevents Uploads from non-Admins
 * Author: TJNowell
 * Author URI: http://tomjn.com
 * Version: 1.0
 */

function tomjn_only_upload_for_admin( $file ) {
    if ( ! current_user_can( 'manage_options' ) ) {
        $file['error'] = 'You can't upload images without admin privileges!';
    }
    return $file;
}
add_filter( 'wp_handle_upload_prefilter', 'tomjn_only_upload_for_admin' );