If you want to fetch posts with thumbnail images there’s a simple trick you can use on your WP query. The trick is to include this simple line of code in your $args array:
'meta_key' => '_thumbnail_id'
Let’s say you want the last 8 published posts that have a featured image set. Simply set your arguments like this and wordpress will do the magic:
$args = array( 'post_type' => 'post', 'order' => 'DESC', 'orderby' => 'date', 'post_status'=>'publish', 'posts_per_page' => 8, 'meta_key' => '_thumbnail_id' );
This trick will allow you to only show posts that have featured image attached to them in your site. May be useful for a homepage where you want to always have a few graphics filling the fold.