http://kachibito.net/wordpress/custom/how-to-add-your-widget.html

class MyWidgetItem extends WP_Widget {
	function MyWidgetItem() {
    	parent::WP_Widget(false, $name = 'リンク先バナー');
    }
    function widget($args, $instance) {
        extract( $args );
        $title = apply_filters( 'widget_title', $instance['title'] );
        $url = apply_filters( 'widget_url', $instance['url'] );
	$img = apply_filters( 'widget_img', $instance['img'] );
    	?>
        <li <?php echo 'class="banner"'; ?> >
           <a href="<?php echo $url; ?>" target="_blank"><img src="<?php echo $img; ?>"></a>
        </li>
        <?php
    }
    function update($new_instance, $old_instance) {
	$instance = $old_instance;
	$instance['title'] = strip_tags($new_instance['title']);
	$instance['url'] = strip_tags($new_instance['url']);
	$instance['img'] = strip_tags($new_instance['img']);
        return $instance;
    }
    function form($instance) {
        $title = esc_attr($instance['title']);
        $url = esc_attr($instance['url']);
	$img = esc_attr($instance['img']);
        ?>
        <p>
          <label for="<?php echo $this->get_field_id('title'); ?>">
          <?php _e('Title:'); ?>
          </label>
          <input class="widefat" id="<?php echo $this->get_field_id('title'); ?>" name="<?php echo $this->get_field_name('title'); ?>" type="text" value="<?php echo $title; ?>" />
        </p>

        <p>
           <label for="<?php echo $this->get_field_id('url'); ?>">
           <?php _e('URL:'); ?>
           </label>
            <input class="widefat" id="<?php echo $this->get_field_id('url'); ?>" name="<?php echo $this->get_field_name('url'); ?>" type="text" value="<?php echo $url; ?>" />
        </p>
        <p>
           <label for="<?php echo $this->get_field_id('img'); ?>">
           <?php _e('img:'); ?>
           </label>
            <input class="widefat" id="<?php echo $this->get_field_id('img'); ?>" name="<?php echo $this->get_field_name('img'); ?>" type="text" value="<?php echo $img; ?>" />
        </p>
	<?php if ( $img ) :?>
	<img src="<?php echo $img; ?>">
	<?php endif;?>
        <?php
    }
}
add_action('widgets_init', create_function('', 'return register_widget("MyWidgetItem");'));