Hi,
I'm providing some code for showing images like thumbnails.try once
<asp:datalist id="DataList1" runat="server" repeatcolumns="4" repeatdirection="Horizontal" xmlns:asp="#unknown">
<HeaderTemplate >
</HeaderTemplate>
<itemtemplate>
<table width="50" align="center">
<tr>
<td><img src="<%#Eval("imgpath") %>" width="50" height="50" /></td>
</tr>
</table>
</itemtemplate>
<footertemplate>
</footertemplate>
</asp:datalist>
Here you'll see 4 columns and rows depends on your count of images
you can adjust the size of image by setting width and height properties
All the Best