almost production version

<?php
function bg_image_plugin_loaded() {
        if(class_exists('Simple_Image_Widget')){
                add_action( 'widgets_init', 'bg_image_widgets_init' );
        } else {
                add_action('admin_notices', 'bg_image_notices');
        }
}
add_action('plugins_loaded', 'bg_image_plugin_loaded');

function bg_image_widgets_init() {
        class BG_Image_Widget extends Simple_Image_Widget {

                function __construct() {
                        $widget_options = array( 'classname' => 'widget_bg_image', 'description' => 'A widget to add images as decorative background patterns.' );
                        parent::__construct( 'bg-image', 'BG Image Widget', $widget_options );

                        add_action('simple_image_widget_field-height', array($this, 'bg_field_height'), 10, 2);
                        add_filter('simple_image_widget_instance', array($this, 'bg_update'), 10, 4);
            //add_filter('simple_image_widget_output', 'bg_image_widget_output');

                }

                function form_fields(){
                        return array('height');
                }

                function bg_field_height($instance) {
                        ?>
                        <p>
                                <label for="<?php echo $this->get_field_id( 'height' ); ?>"><?php _e( 'Height:', 'bg-image-widget' ); ?></label>
                <input type="text" name="<?php echo $this->get_field_name( 'height' ); ?>" id="<?php echo $this->get_field_id( 'height' ); ?>" value="<?php echo esc_attr( $instance['height'] ); ?>" class="widefat">
                        </p>
                        <?php
                }

                function bg_update($instance, $new_instance, $old_instance, $id_base) {
                        // @todo Sanitize custom values.
                        return $instance;
                }

        function render($args, $instance){

            $src = wp_get_attachment_image_src( $instance['image_id'], 'full');

            if ( ! empty( $instance['image_id'] ) ) {
                $output = '<div class="pattern-widget" id="bg-'.urlencode($instance['title']).'" style="background-image:url('.$src[0].'); height:'.$instance['height'].'px;">';
                $output .= '</div><!---->';
            }else{
                $output = '<div class="hidden error"><p>There is no image for this, please pick an image first!</p></div>';
            }
            return $output;
        }

        }

        // Register the widget.
        register_widget( 'BG_Image_Widget' );
}

function bg_image_notices(){
        global $pagenow;
        if($pagenow == 'widgets.php'){
                echo '<div class="updated">No Simple Image Widget Class for BG Image Widget to Use</div>';
        }
}