Gel all image from certain post type

Try this code in your template.

$query = new WP_Query( array( 'post_type' => 'custom-post', 'posts_per_page' => -1 ) );
if( $query->have_posts() ){
    while($query->have_posts()){
        $query->the_post();
        $image_query = new WP_Query( array( 'post_type' => 'attachment', 'post_status' => 'inherit', 'post_mime_type' => 'image', 'posts_per_page' => -1, 'post_parent' => get_the_ID() ) );
        while( $image_query->have_posts() ) {
            $image_query->the_post();
            echo wp_get_attachment_image( get_the_ID() );
        }
    }
}

Replace custom-post with you custom post type.