adding alt attribute to testimonials images

Hi,
I am using Testimonials plugin.
Unfortunately, this plugin does not provide the alt attribute.
How can I modify the plugin to do this?
Here is an extract of the plugin:

while ( $the_query->have_posts() ) : $the_query->the_post();

        $client_name_value =get_post_meta(get_the_ID(), 'Client Name', true);

		$author_value = get_post_meta(get_the_ID(), '_author', true);

		$author_details_value = get_post_meta(get_the_ID(), '_author_details', true);

 		$url =wp_get_attachment_thumb_url( get_post_thumbnail_id(@$post->ID) );

        $result .='<li>'; // Start Slide

		$result .='<div class="cbp-qtcontent">'; // Start Slide

		$result .='<div class="be-testimonial-wrap">'; // wrapper

		if ( has_post_thumbnail() ) { // check if the post has a Post Thumbnail assigned to it.

		  $result .= '<img  class="be-testi-author-img" src="'.$url.'" />';

		}

Thanks a lot!!!

https://wordpress.org/plugins/hms-testimonials/