implement only image of latest post in own site

Hello guys, i’m new to wordpress and i’m trying to integrate it in my existing site.

To do so i’m using the following code:

<?php
$args = array( 'numberposts' => 1 );
$lastposts = get_posts( $args );
foreach($lastposts as $post) : setup_postdata($post); ?><?php the_content(); ?><?php endforeach; ?>

This does the trick, but now i would like to only have posts of a particular categorie be shown. So i tried to add 'category' => 3 to the $args = array line. But if i do so, nothing shows up (and there are posts in this categorie). Any ideas why this is?

Also, in this case, i would like to only have the image of the post be shown, so no title, date (i already removed <?php the_title(); ?> and <?php the_date(); ?> so that wont show up), but the permalink and excerpt arte still there and don’t have their own <?php> tag in the code as seen above.. So can this be done?

The site im talking about http://www.moschalkx.nl/index1.php

Hope you guys can make something of it!