Sample WP_Query

wp_query.php

<?php
			$args = array(
				'post_type' => 'slider',
				'posts_per_page' => -1,
			);
			$slider_query = new WP_Query( $args );
			?>
				<?php if ( $slider_query->have_posts() ): ?>

					<?php while ( $slider_query->have_posts() ): $slider_query->the_post(); ?>
						<?php if ( has_post_thumbnail() ): ?>
							<?php the_post_thumbnail( 'slider' ); ?>
						<?php endif; ?>
					<?php endwhile; ?>

				<?php endif; ?>
<?php wp_reset_postdata(); ?>