try this....
<% for(int i = 0; i < dt5.Rows.Count; i++)
{
%>
<div id="Div1" class="image_stack" style="margin-left:600px" runat="server" >
<img class="stackphotos photo1" src="~/Images/3.jpg" clientidmode="static" runat="server" />
<img class="stackphotos photo2" src="~/Images/4.jpg" clientidmode="static" runat="server"/>
<img class="stackphotos photo3" src="~/Images/5.jpg" clientidmode="static" runat="server"/>
</div>
<pre>
<% Img1.Src="~/Images/3.jpg";
Img2.Src="~/Images/4.jpg";
Img3.Src="~/Images/5.jpg"; %>
<br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br />
<% } %>