<table border=1>
<tr>
<td width="100">
<center>
<img name="chimg" src="1.gif"/>
</center>
</td>
</tr>
<tr>
<td><center>
<a href="javascript:preimg();">◀</a>
<a href="javascript:nextimg();">▶</a>
</center></td>
</tr>
</table>
</form>

<script>
first =0;

function preimg(){
if( first<=0)return;
else{
first --;
document.test.chimg.src=first+".gif";
}
}
function nextimg(){
first++;
document.test.chimg.src=first+".gif";
}
</script>

</form>