Before I begin, it is worth highlighting that I am using an ordered list of links, and the hidden image is a background image on the <ol> element. I will take the following approach:

1. Store the url of the hidden image
2. Add an element to each list item, which will be used to display the hidden portion of the image
3. then add the click events to the numbered square and the reveal element

So the first few lines set a few variables, allowing us to cache some jQuery objects for later use.

$gridWrap = $('#reveal_grid');
$revealImg = $gridWrap.css('background-image');
$gridWrap.css('background', 'transparent')
$sqrs = $('#reveal_grid li');

We then loop through the links in the list and add the elements, add some CSS rules to setup the hide/reveal.

$('#reveal_grid li a').each(function() {
	$num = $(this);
	$bgPos = $num.css('backgroundPosition');
	$num.parent('li').append(
		$('').css({
			backgroundImage: $revealImg,
			backgroundPosition: $bgPos,
			height:'0',
			marginTop: ($num.height()/2) + 'px',
			opacity:'0.2'
		})
	);
});

Next we need to add the click events, which just adds the flip animation that was based upon the original by Aayush Shastri:

$sqrs.delegate('a', 'click', function() {
	$sqr = $(this);
	$height = $sqr.height();
	$width = $sqr.width();
	$sqr.stop()
	.animate({
		width: $width + 'px',
		height: '0',
		marginTop: ($height/2) + 'px',
		opacity: '0.2'
	}, 500, function() {
	$sqr.next('.reveal')
		.stop()
		.animate({
			width: $width + 'px',
			height: $height + 'px',
			marginTop: '0',
			opacity: '1'
		}, 500);
	});
	return false;
});
$sqrs.delegate('span', 'click', function() {
	$sqr = $(this);
	$height = $sqr.height();
	$width = $sqr.width();
	$sqr.stop()
	.animate({
		width: $width + 'px',
		height: '0',
		marginTop: ($height/2) + 'px',
		opacity: '0.2'
	}, 500, function() {
		$sqr.prev('a')
		.stop()
		.animate({
			width: $width + 'px',
			height: $height + 'px',
			marginTop: '0',
			opacity: '1'
		}, 500);
	});
	return false;
});

I am aware that this code could be streamlined somewhat, but it made sense to structure it like this for demonstration purposes. In terms of performance, it seems fine, so I will leave as is for the time being.