<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false">
<Columns>
<asp:TemplateField>
<HeaderTemplate>
image
</HeaderTemplate>
<ItemTemplate>
<asp:Image CssClass="Image1" ImageUrl='<%# Eval("Image") %>' runat="server" />
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
////////////////////////////////////////////////////////////////////////////////////////
use this jquery for display image on mousehover
$('#<%=GridView1.ClientID %>').find('img[class$="Image1"]').mouseover(function () {
$('#<%=GridView1.ClientID %>').find('img[class$="Image1"]').width(500);
$('#<%=GridView1.ClientID %>').find('img[class$="Image1"]').height(500);
});
$('#<%=GridView1.ClientID %>').find('img[class$="Image1"]').mouseout(function () {
$('#<%=GridView1.ClientID %>').find('img[class$="Image1"]').width(100);
$('#<%=GridView1.ClientID %>').find('img[class$="Image1"]').height(100);
});
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put your dialog box open code in mouseover and mouseout event...