<?php
	$category_ID = 25;

	global $wpdb;

	// get the first 18 posts with an attachment from the database
	$myposts = $wpdb->get_results("
	  SELECT *
	  FROM $wpdb->posts
	  LEFT JOIN $wpdb->term_relationships ON ($wpdb->posts.ID = $wpdb->term_relationships.object_id)
	  LEFT JOIN $wpdb->term_taxonomy ON ($wpdb->term_relationships.term_taxonomy_id = $wpdb->term_taxonomy.term_taxonomy_id)
	  WHERE post_status = 'publish'
	  AND ID IN (
	    SELECT DISTINCT post_parent
	    FROM $wpdb->posts
	    WHERE post_parent > 0
	    AND post_type = 'attachment'
	    AND post_mime_type IN ('image/jpeg', 'image/png')
	  )
	  AND $wpdb->term_taxonomy.taxonomy = 'category'
	  AND $wpdb->term_taxonomy.term_id = '" . $category_ID . "'
	  ORDER BY RAND() LIMIT 0,18
	");

	if($myposts){
		foreach($myposts as $post) {
			setup_postdata($post);
			$images = get_children(array(
			  'post_parent' => $post->ID,
			  'post_type' => 'attachment',
			  'post_mime_type' => 'image',
			  'orderby' => 'rand'
			));

			if(!empty($images)){
				$ids = array_keys($images);
				// link attachement to post
				echo '<a href="' . get_permalink($post->ID) . '" >' . wp_get_attachment_image($ids[0], 'thumbnail') . '</a>';
			}

		}
		wp_reset_postdata();
	}
?>