Get post thumbnail in WP_Query

Do you mean the featured image? Use wp_get_attachment_image – Here is the code that I use in my plugin

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

 $image_id = get_post_thumbnail_id();

 $imagesize="thumbnail";

 $image_url = wp_get_attachment_image_src($image_id, $imagesize, true);

//do something

 endwhile;

You can also use wp_get_attachment_url( $id );