なんか入り乱れてるけど、とにかく全部貼っちゃいます
>>539
ヘッダはこんな感じ

class TForm1 : public TForm
{
__published:
TImage *Image1;
private:
TWndMethod OldImageWndProc;
void __fastcall ImageWndProc(TMessage& Msg);
public:
__fastcall TForm1(TComponent* Owner);
__fastcall ~TForm1();
};

cppソースはこんな感じ

__fastcall TForm1::TForm1(TComponent* Owner) : TForm(Owner)
{
OldImageWndProc = Image1->WindowProc;
Image1->WindowProc = ImageWndProc;
}

__fastcall TForm1::~TForm1()
{
Image1->WindowProc = OldImageWndProc;
}

void __fastcall TForm1::ImageWndProc(TMessage& Msg)
{
if (Msg.Msg == CM_MOUSEENTER)
ShowMessage("Mouse Enter");

OldImageWndProc(Msg);
}