Gridview with Image thumbnail and click Image open details using vb code behind in asp.net

Here is my default.aspx code
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<asp:gridview id="GridView1" runat="server" xmlns:asp="#unknown">
DataSource='<%# GetData() %>' AutoGenerateColumns="False" AllowPaging="True"
BorderWidth="1px" BackColor="White" CellPadding="3" BorderStyle="None"
BorderColor="#CCCCCC" Font-Names="Arial">
<footerstyle forecolor="#000066" backcolor="White">
<pagerstyle forecolor="#000066" horizontalalign="Left">
BackColor="White">
<headerstyle forecolor="White" font-bold="True">
BackColor="#006699">
<columns> <asp:boundfield headertext="Num" datafield="NumID">
<itemstyle horizontalalign="Center" verticalalign="Middle">


<asp:templatefield headertext="Status" itemstyle-horizontalalign="Center">
<itemtemplate>
<asp:image id="completedImage" runat="server" imageurl="<% # completedImage() %>" alternatetext="">



<asp:boundfield headertext="Title" datafield="Title" sortexpression="Title">
<asp:boundfield headertext="Date Added" datafield="DateAdded" dataformatstring="{0:d}"> <itemstyle horizontalalign="Center">

<asp:boundfield headertext="ISO File" datafield="ISOFile">
<itemstyle horizontalalign="Center">

<asp:boundfield headertext="Mobile" datafield="MBFile">
<itemstyle horizontalalign="Center">

<asp:boundfield headertext="HD File" datafield="HDFile">
<itemstyle horizontalalign="Center">

<asp:imagefield headertext="Picture" dataimageurlfield="PictureURL">
ControlStyle-Height="150" ControlStyle-Width="105"
alternatetext="Movie Photo"
NullDisplayText="No image on file." readonly="true">
<controlstyle height="150px" width="105px">

<selectedrowstyle forecolor="White" font-bold="True" backcolor="#669999">
<rowstyle forecolor="#000066">




Here is the default.aspx.vb codes:
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Public Function completedImage() As String
completedImage = "\myFiles\KidPhoto.jpg"
result = "\images\yellow.gif"
Return result
End Function


I am unable to define and access to change the jpg photo and gif photo in code behind.
The gif photo show up fine but does not allow me to click photo and bring up the bigger jpg photo. Please help..