This should work for you:
// grab all public post types
$post_types = get_post_types( array('public' => true), 'names' );
// loops through each post type
foreach( $post_types as $type ) {
// setup the query
$query_args = array(
'post_type' => $type,
'posts_per_page' => 1
);
// perform the query
$items = get_posts( $query_args );
// check if we have found anything
if( $items ) {
// loop through the items
foreach( $items as $item ) {
// show the post thumbnail
echo get_the_post_thumbnail( $item->ID, 'thumbnail' );
}
}
}