If you don't want to change data structure, or query... and want to do it in code front, you can do it like this (though it's cleaner to do in ItemDataBound):
<asp:Image ID="imageControl" runat="server" Width="100" ImageUrl='<%# DataBinder.Eval(Container.DataItem, "FilePath") != null ? Eval("FilePath") : Eval("PicURL") %>'></asp:Image>