If it's acceptable to you to hard code the ImageUrl property, then this should work. Notice the ~/ at the start of the paths to get the app root folder:
<asp:HyperLink ID="BuyLink" runat="server"
NavigateUrl='<%# "~/StoreModules/AddToCart.aspx?PID=" + DataBinder.Eval(Container.DataItem, "ProductID").ToString() %>'
ImageUrl="~/CommonImagesFolder/buy_btn.gif" />