HI
If you are working on local machine then you just append the path(C:\\folder) with dynamic data like
<asp:Image ID="Image3" runat="server"ImageUrl='C:\\barcode<%# Eval("AppNo")+ %>.gif' Width="110px" Height="30px" />
if you are working on server then you have to make the folder virtual directory and then append it on the path like
<asp:Image ID="Image3" runat="server"ImageUrl='/barcode/<%# Eval("AppNo")+ %>.gif' Width="110px" Height="30px" />
Hope you got it.
Regard
Ehsan