>テーブルに入れる前提で、
ではレイヤーではなく、テーブルでやってみたらいかがでしょう?
<html>
<head>
<title>サムネイル表示テスト</title>
<script type="text/javascript">
function onImg(img){
myImg.src = img
zoomT.style.display = "block"
baseT.style.display = "none"
}
function offImg(){
zoomT.style.display = "none"
baseT.style.display = "block"
}
</script>
</head>
<body>
<table border="3" id="zoomT" width="200" onmouseout="offImg()" style="display:none;" align="center">
<tr>
<td><img id="myImg" width="100%"></td>
</tr>
</table>
<table border="0" id="baseT" style="display:block;" align="center">
<tr>
<td><img src="dummy.gif" width="50" height="50" onmouseover="onImg(this.src)"></td>
<td><img src="dummy2.gif" width="50" height="50" onmouseover="onImg(this.src)"></td>
<td><img src="dummy3.gif" width="50" height="50" onmouseover="onImg(this.src)"></td>
<td><img src="dummy4.gif" width="50" height="50" onmouseover="onImg(this.src)"></td>
<td><img src="dummy5.gif" width="50" height="50" onmouseover="onImg(this.src)"></td>
</tr>
</table>
</body>
</html>