Problem with Event Handler for component created dynamically at runtime

I must be doing something wrong.  When I try to compile, I get the following
linker error:
[Linker Error] Unresolved external '__fastcall
TForm1::MyOnMouseMove(System::TObject *, System::Set<Classes::Classes__1, 0,
6>, int, int)' referenced from THUMBNAILSPROJ.OBJ (shortened version)

Here's my code setting the OnMouseMove:

                TImage *img = (TImage *)(imgList->Items[tmpindex]);
                // Set img properties
                img->Parent = Form1;
                img->OnMouseMove = MyOnMouseMove;

Here's the function for MyOnMouseMove:

void __fastcall MyOnMouseMove(TObject *Sender, TShiftState Shift, int X, int
Y)
{}

Here's the header file definition under the public: area for the form:

public:  // User declarations
        void __fastcall MyOnMouseMove(TObject *Sender, TShiftState Shift,
int X, int Y);
        __fastcall TForm1(TComponent* Owner);

Quote
};

What am I doing wrong?

- Paul