procedure TForm1.Button1Click(Sender: TObject);
var
  i, j: integer;
  Pt, St: TPoint;
begin
   { Define the starting point = left-top corner of image 'RightImg' }
   { 10 pixels from the left-top edges }
   St.x := RightImg.Left + 10;
   St.y := RightImg.Top + 10; 
   { draw 25 random brush lines within the boundaries of the image }
   for j := 1 to 25 do 
   begin 
     { required to draw one brush line }
     for i := 1 to 2 do
     begin  
      { effectively 10 pixels from the right-bottom edges }
      Pt.X := St.X + random(RightImg.Width - 20);
      Pt.Y := St.Y + random(RightImg.Height - 20);
      { Convert Pt to screen coordinates relative to the 65535 mouse-steps across the screen }
      Pt := ClientToScreen(Pt);
      Pt.x := Round(Pt.x * (65535 / Screen.Width));
      Pt.y := Round(Pt.y * (65535 / Screen.Height));
      { Simulate the mouse move }
      Mouse_Event(MOUSEEVENTF_ABSOLUTE or
                  MOUSEEVENTF_MOVE,
                  Pt.x, Pt.y, 0, 0);
      { Simulate the left mouse button down }
      if i = 1 then Mouse_Event(MOUSEEVENTF_ABSOLUTE or
                  MOUSEEVENTF_LEFTDOWN,
                  Pt.x, Pt.y, 0, 0);
      { Simulate the left mouse button up }
      if i = 2 then Mouse_Event(MOUSEEVENTF_ABSOLUTE or
                  MOUSEEVENTF_LEFTUP,
                  Pt.x, Pt.y, 0, 0);
    end; 
    { optional when 'Application.ProcessMessages' is used }
    RightImg.Update;
    { optional: to watch the painting take place }
    Application.ProcessMessages; 
  end;
end;

  RightImg.BrushTool.BrushShape := iecsCircle;
  RightImg.BrushTool.BrushFill := iebfTextured;
  RightImg.BrushTool.BrushSize := 50;
  RightImg.BrushTool.BrushColor := clRed;
  RightImg.MouseInteract := [miBrushTool];
  RightImg.BrushTool.PaintMode := iepmContinuous;