[deltas]

Я обычно делаю так -

Код:

...
var
  Form1: TForm1;
CursorPos : TPoint;
  FormPos : TPoint;
   Drag : boolean;
implementation
{$R *.dfm}
...

procedure TForm1.OnIdle(Sender: TObject; var Done: boolean);
var Point: TPoint;
begin
  if Drag then
  begin
    GetCursorPos(Point);
    Left := FormPos.X + Point.X - CursorPos.X;
    Top := FormPos.Y + Point.Y - CursorPos.Y;
  end;
end;

procedure TForm1.Image1MouseDown(Sender: TObject;
  Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
begin
 if Button = mbLeft then
  begin
    Drag := True;
    FormPos.X := Left;
    FormPos.Y := Top;
    GetCursorPos(CursorPos);
  end;
end;

procedure TForm1.Image1MouseUp(Sender: TObject;
  Button: TMouseButton; Shift: TShiftState; X, Y: Integer);
begin
 if Button = mbLeft then Drag := False;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin
Application.OnIdle := OnIdle;
end;