hi,
find a default image you want to show, place it under your web site roor, lets say your default image name is
default.jpg
, now use this code below in your page
<asp:Image ID="Image1" Width="75%" Height="75%" runat="server" ImageUrl='<%# (String.IsNullOrEmpty(Eval("Photo").ToString()) ? Eval("Photo"):"~/default.jpg") %>' />
Thats it..