Let try to use jQuery to achieve this:
add the jquery code to the head section:
<head id="Head1" runat="server">
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.10.2/jquery.min.js"></script>
<script type="text/javascript">
$(document).ready(function () {
$(".Image1").click(function () {
// traversing your current table structure
var $checkbox = $(this).parent().parent().parent().next().children().children();
$checkbox.prop("checked", !$checkbox.prop("checked"));
});
});
</script>
</head>
Modify the html a bit:
<asp:DataList ID="DataList1" runat="server" RepeatColumns="4" RepeatDirection="Horizontal" ClientIDMode="Static">
<ItemTemplate>
<img src='<%#Eval("ThumbfileLink") %>' height="170px" width="270px" class="Image1" />
</asp:DataList>
Hope it helps. Good luck.