UI is not updated instead of using ObservableCollection

how does the UI know Technicaly to make it green or transparent ?
could you elaborate more on your problem from technical points with some relavent classes 
?

I am not changing the tag.And why do we need to do so? because tags are just to determine which item from the list has been selected.


The code snippet:-
partListPhoneControl.myCart.MouseLeftButtonUp += (sender, e) =>
                                {
                                    Image img = (Image)sender;
                                    Object obj = img.Tag;
                                    int val = (int)obj;
                                    if (myCollection[val].isAddedToCart() == true)
                                    {
                                        partListPhoneControl.myCart.Source = new BitmapImage(new                         Uri("/images/shopping_cart_selected.png", UriKind.Relative));
                                           
                                     }
                                 else
                                 {                        
                                         partListPhoneControl.myCart.Source = new BitmapImage(new                                 Uri("/images/shopping_cart_default.png", UriKind.Relative));
                                 }

 }