WordPress image preloading

preload-images.js

/**
 * Image preloader
 *
 * @link http://engineeredweb.com/blog/09/12/preloading-images-jquery-and-javascript
 */
var cache = [];
// Arguments are image paths relative to the current page.
function pilau_preload_images() {
	var args_len, i, cache_image;
	args_len = arguments.length;
	for ( i = args_len; i--;) {
		cache_image = document.createElement('img');
		cache_image.src = arguments[i];
		cache.push( cache_image );
	}
}

preload-images.php

<?php

/**
 * Add an image for preloading
 *
 * All images must be added using this function before the header is output
 *
 * @param	mixed	$img	Either a string path, or a WP attachment object (the guid will be used)
 * @return	void
 **/
function pilau_add_image_for_preloading( $img ) {
	global $pilau_preload_images;
	if ( ! isset( $pilau_preload_images ) ) {
		$pilau_preload_images = array();
	}
	if ( is_object( $img ) && property_exists( $img , 'guid' ) ) {
		$pilau_preload_images[] = $img->guid;
	} else if ( is_string( $img ) ) {
		$pilau_preload_images[] = $img;
	}
}


/**
 * Set up images for preloading
 **/
add_action( 'wp_head', 'pilau_images_preload', 100 );
function pilau_images_preload() {
	global $pilau_preload_images;
	if ( isset( $pilau_preload_images ) && ! empty( $pilau_preload_images) ) {
		?>
		<script type="text/javascript">
			pilau_preload_images( "<?php echo implode( '", "', $pilau_preload_images ); ?>" );
		</script>
		<?php
	}
}

https://wordpress.org/plugins/preload-images/
this one help which same code based a plugin.