Jonai, probé el código antes de ponértelo y funcionaba. Inténtalo con ese código sólo:
<html>
<head>
<script type="text/javascript">
browserName = navigator.appName;
browserVer = parseInt(navigator.appVersion);
ns3up = (browserName == "Netscape" && browserVer >= 3);
ie4up = (browserName.indexOf("Microsoft") >= 0 && browserVer >= 4);
function doPic(imgName) {
if (ns3up || ie4up) {
imgOn = ("" + imgName);
document.images["mainpic"].src = imgOn;
document.links["mainLink"].href = imgOn
}
}
</script>
</head>
<body>
<div>
<a name="mainLink" href="1.gif"><img src="1.gif" name="mainpic"></a>
</div>
<a href="javascript
:doPic('1.gif');">1</a>
<a href="javascript
:doPic('2.gif');">2</a>
<a href="javascript
:doPic('3.gif');">3</a>
<a href="javascript
:doPic('4.gif');">4</a>
</body>
</html>