get_posts returns all posts

I’m trying to display an image as a link for a category, and I’d prefer that image to be one that is attached to a post in that category. Currently it just displays the same image for all of them.

This is what I have right now

<?php $args = array( 'post_type' => 'post' );
$categories=get_categories($args);
$thumbURL;
$posts;
$id;?>

<?php foreach($categories as $category) : ?>

	<?php
	$args = array(
		'suppress_filters' => false,
		'post_type' => 'attachment',
		'numberposts' => -1,
		'category' =>$category->term_ID,
		'order'    => 'ASC',
		);
	$posts=get_posts( $args );

	if ($posts){
		foreach($posts as $post){
			if ( wp_attachment_is_image() ){
				if( $thumbURL=wp_get_attachment_thumb_url() ){
					break;
				}
			}
		}
	}
	?>
		<a class="categoryNav" href="<?php echo get_category_link($category->term_id); ?>" title="<?php echo $category->name; ?>">
		<img class="categoryNav" src="<?php echo $thumbURL; ?>" alt="<?php echo $category->name; ?> : <?php echo $category->description; ?>" />
	</a>

<?php endforeach ?>

I’m pretty new to PHP and wordpress, so any feedback would probably be helpful to me.