Please check below code to get all posts 

    $args = array(
		'posts_per_page' => -1,
		'post_type' => 'post',
		'orderby' => 'date',
		'order' => 'DESC',
	);

	query_posts($args);
	while (have_posts()): the_post();
    the_title();
    the_excerpt();
    get_the_post_thumbnail_url($post->ID);
    endwhile;
    wp_reset_query();

Please try above code. 
Note: `get_the_post_thumbnail_url();` will return featured image URL. so you can directly add into `img` tag like

    <img src="<?php echo get_the_post_thumbnail_url(); ?>">