Show image in excerpt post. Image not displaying

If you add support for Post Thumbnails, you can use its own function instead of get_post_meta(), see codex for more information – http://codex.wordpress.org/Post_Thumbnails#Function_Reference

Use following code in your theme:

<div class="postcont>        
<?php
    if ( has_post_thumbnail() ) {
        the_post_thumbnail();
    }
    the_excerpt();
?>
</div>

EDIT

And using the OP’s code:

$args = array( 'numberposts' => 5, 'category' => 3,4,5,6,7 );
$posts = query_posts( $args . '&orderby=date&order=desc' );
foreach( $posts as $post ) :    setup_postdata($post); ?>
<li>
    <h2><a href="https://wordpress.stackexchange.com/questions/28967/<?php the_permalink(); ?>"><?php the_title(); ?></a></h2>
    <div class="postcont">
        <?php 
        if ( has_post_thumbnail() ) {
            the_post_thumbnail();
        }
        the_excerpt();  ?>
    </div>
</li>
<?php endforeach; ?>