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uses
ShellApi;
...
  private
    Procedure WMDROPFILES (Var aMsg: tMessage);  message WM_DROPFILES;
...
Procedure TForm1.WMDROPFILES (Var aMsg: tMessage);
...
procedure TForm1.FormDragOver(Sender, Source: TObject; X, Y: Integer;
  State: TDragState; var Accept: Boolean);
begin
showmessage('test');
//image1.visible := False;
//image2.visible := True;
end;                    

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type
  TForm2 = class(TForm)
    Memo1: TMemo;
    procedure FormCreate(Sender: TObject);
  private
    { Private-Deklarationen }
        procedure WMDropFiles(var Msg: TWMDropFiles); message WM_DROPFILES;
  public
    { Public-Deklarationen }
  end;

var
  Form2: TForm2;

implementation
uses ShellAPI;
{$R *.dfm}
procedure TForm2.FormCreate(Sender: TObject);
begin
  DragAcceptFiles(Handle, True);
end;

procedure TForm2.WMDropFiles(var Msg: TWMDropFiles);
var
  CFileName: array [0 .. MAX_PATH] of Char;
  FileName:String;
  i,DroppedFileCount:Cardinal;
begin
  try
   DroppedFileCount := DragQueryFile(Msg.Drop, $FFFFFFFF, nil, 0);
   for I := 0 to DroppedFileCount - 1 do
      begin
        SetLength(FileName,  DragQueryFile(Msg.Drop, i, nil, 0));
        DragQueryFile(Msg.Drop, i, PChar(FileName), Length(FileName) +1 );
        Memo1.Lines.Add(FileName);
      end;
  finally
    Msg.Result := 0;
    DragFinish(Msg.Drop)
  end
end;