Il tuo script è un casino da guardare. Ti posso però dire come modificare uno di quelli che ti ho consigliato per lo scroll orizzontale. Prendi questo: https://developer.mozilla.org/@api/d...ed-gallery.zip e sostituisci il contenuto del file css-checked-gallery.html con questo codice:

codice:
<!doctype html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>CSS input:checked gallery example - MDN</title>
<style type="text/css">
#gallery-container {
	position: relative;
	width: 700px;
	height: auto;
	padding-top: 350px;
	margin-left: auto;
	margin-right: auto;
	text-align: center;
	border: 1px #aaaaaa solid;
}

img.gallery-fullsize {
	position: absolute;
	top: 15px;
	left: 150px;
	display: none;
}

div.gallery-item {
	display: inline-block;
}

input.gallery-selector {
	display: none;
}

label.gallery-label {
	cursor: pointer;
}

input.gallery-selector:checked ~ img.gallery-fullsize {
	display: block;
}

#gallery-scroll {
	height: 100px;
	width: auto;
	overflow: auto;
}

#gallery-previews {
	height: 75px;
	width: 900px;
}
</style>
</head>

<body>
<h1>CSS input:checked gallery</h1>
<div id="gallery-container">
	<div id="gallery-scroll">
		<div id="gallery-previews">
			<div class="gallery-item">
				<label for="gallery-item1" class="gallery-label">[img]images/1s.jpg[/img]</label>
				<input type="radio" name="gallery-list" class="gallery-selector" value="1.jpg" id="gallery-item1" />
				[img]images/1.jpg[/img]
			</div>
			<div class="gallery-item">
				<label for="gallery-item2" class="gallery-label">[img]images/2s.jpg[/img]</label>
				<input type="radio" name="gallery-list" class="gallery-selector" value="2.jpg" id="gallery-item2" />
				[img]images/2.jpg[/img]
			</div>
			<div class="gallery-item">
				<label for="gallery-item3" class="gallery-label">[img]images/3s.jpg[/img]</label>
				<input type="radio" name="gallery-list" class="gallery-selector" value="3.jpg" id="gallery-item3" />
				[img]images/3.jpg[/img]
			</div>
			<div class="gallery-item">
				<label for="gallery-item4" class="gallery-label">[img]images/4s.jpg[/img]</label>
				<input type="radio" name="gallery-list" class="gallery-selector" value="4.jpg" id="gallery-item4" />
				[img]images/4.jpg[/img]
			</div>
			
			<div class="gallery-item">
				<label for="gallery-item5" class="gallery-label">[img]images/1s.jpg[/img]</label>
				<input type="radio" name="gallery-list" class="gallery-selector" value="1.jpg" id="gallery-item5" />
				[img]images/1.jpg[/img]
			</div>
			<div class="gallery-item">
				<label for="gallery-item6" class="gallery-label">[img]images/2s.jpg[/img]</label>
				<input type="radio" name="gallery-list" class="gallery-selector" value="2.jpg" id="gallery-item6" />
				[img]images/2.jpg[/img]
			</div>
			<div class="gallery-item">
				<label for="gallery-item7" class="gallery-label">[img]images/3s.jpg[/img]</label>
				<input type="radio" name="gallery-list" class="gallery-selector" value="3.jpg" id="gallery-item7" />
				[img]images/3.jpg[/img]
			</div>
			<div class="gallery-item">
				<label for="gallery-item8" class="gallery-label">[img]images/4s.jpg[/img]</label>
				<input type="radio" name="gallery-list" class="gallery-selector" value="4.jpg" id="gallery-item8" />
				[img]images/4.jpg[/img]
			</div>
		</div>
	</div>
</div>
</body>
</html>
Altrimenti... buona fortuna