widget coding issue in multisite

I have a select widget in a multisite installation. The user selects one of several images and adds a link to a page. I hard coded links to the images. It works, but when I changed from my development server to the actual server, of course the domain and url changed and I had to hard-code everything again and lost all the widgets.

That’s okay for one site, but won’t work once the other sites are in place (I anticipate working with the site on the development server again). First time doing this coding and I can’t figure out how to make it dynamic. Have tried a couple of things and they don’t work.

The images are in the theme folder and the theme is “rainbow.” Here’s the code (some options omitted to make this shorter):

// Output the admin options form
function form($instance) {
?>
<p>
<label for="<?php echo $this->get_field_id('title'); ?>"><?php _e('Title:'); ?></label>
<input id="<?php echo $this->get_field_id('title'); ?>" name="<?php echo $this->get_field_name('title'); ?>" value="<?php echo $instance['title']; ?>" type="text" class="widefat" />
</p>
<p>
<label for="<?php echo $this->get_field_id('image'); ?>">Select image:</label>
<select id="<?php echo $this->get_field_id('image'); ?>" name="<?php echo $this->get_field_name('image'); ?>" class="widefat" style="width:100%;">
<option value="http://localhost/rainbow/wp-content/themes/rainbow/images/widgets/built-to-last.jpg" <?php selected('http://localhost/rainbow/wp-content/themes/rainbow/images/widgets/built-to-last.jpg', $instance['image']); ?>>Built to Last</option>
<option value="http://localhost/rainbow/wp-content/themes/rainbow/images/widgets/come-visit.jpg" <?php selected('http://localhost/rainbow/wp-content/themes/rainbow/images/widgets/come-visit.jpg', $instance['image']); ?>>Come and Visit</option>
</select>
</p>
<p>
<label for="<?php echo $this->get_field_id('linkurl'); ?>"><?php _e('Page you are linking to:'); ?></label>
<input id="<?php echo $this->get_field_id('linkurl'); ?>" name="<?php echo $this->get_field_name('linkurl'); ?>" value="<?php echo $instance['linkurl']; ?>" type="text" class="widefat" />
</p>
<?php
}