If you are trying to show the featured image for each of the posts in the loop, then this code needs to be done inside of the loop
$featured_image = wp_get_attachment_image_src(get_post_thumbnail_id(), 'thumbnail' );
Second, wp_get_attachment_image_src() returns an array https://developer.wordpress.org/reference/functions/wp_get_attachment_image_src/#div-comment-2199
<img src='<?php echo $featured_image[0]; ?>' alt='Photo' class='thumbphoto'>
