Código:
<img id="imgprincipal" src="<? echo $row[5]; ?>" alt="<? echo $row[2]; ?>" onerror="this.src='images/item-pic300.jpg'" width="300" height="301" />
<script language="javascript">
function showimg(url){
document.getElementById('imgprincipal').src='url';
}
</script>
<p align="center">
<img onmouseover="javascript:showimg('<? echo $row[5]; ?>')" src="<? echo $row[5]; ?>" onerror="this.src='images/item-pic50.jpg'" width="50" height="50" />
<img onmouseover="javascript:showimg('<? echo $row[5]; ?>')" src="<? echo $row[6]; ?>" onerror="this.src='images/item-pic50.jpg'" width="50" height="50" />
<img onmouseover="javascript:showimg('<? echo $row[5]; ?>')" src="<? echo $row[7]; ?>" onerror="this.src='images/item-pic50.jpg'" width="50" height="50" />
<img onmouseover="javascript:showimg('<? echo $row[5]; ?>')" src="<? echo $row[8]; ?>" onerror="this.src='images/item-pic50.jpg'" width="50" height="50" />
<img onmouseover="javascript:showimg('<? echo $row[5]; ?>')" src="<? echo $row[9]; ?>" onerror="this.src='images/item-pic50.jpg'" width="50" height="50" />
</p>

