如何在wordpress testemonials插件中添加div？

I am using this plugin for testimonials testimonials and i want to add my custom div foreach testimonial.I found this example for editing view of plugin,but i still dont understand how it work.So how can i add it?

In your theme function.php add the bellow code and change the image url with your image url http://www.subway.com/core/images/topNavBar/subway-logo.png

Code

 add_filter( 'testimonials_widget_testimonial_html', 'my_testimonials_widget_testimonial_html', 10, 5 );

function my_testimonials_widget_testimonial_html( $content, $testimonial, $atts, $is_list = true, $is_first = false, $widget_number = null ) {

    echo'<div class="my-image"><img width="98%" class="custom-image" src="http://www.subway.com/core/images/topNavBar/subway-logo.png" alt="" /></div>';


    return $content;
}