function SelectImg(url){
document.all.item("ProductImg").src=document.getElementById("FileUpload1").value;
}
<asp:FileUpload ID="FileUpload1" runat="server" onchange="SelectImg(this)" />
document.all.item("ProductImg").src=document.getElementById("FileUpload1").value;
}
<asp:FileUpload ID="FileUpload1" runat="server" onchange="SelectImg(this)" />