procedure Spray(Canvas: TCanvas; x, y, r: Integer; Color: TColor);
var
rad, a: Single;
i: Integer;
begin
for i := 0 to 100 do
begin
a := Random * 2 * pi;
rad := Random * r;
Canvas.Pixels[x + Round(rad * Cos(a)), y + Round(rad * Sin(a))] := Color;
end;
end;

procedure TForm1.Image1MouseMove(Sender: TObject; Shift: TShiftState; X,
Y: Integer);
begin
if ssLeft in Shift then Spray(Image1.Canvas, x, y, 40, clRed);
end;

procedure TForm1.Image1MouseDown(Sender: TObject; Button: TMouseButton;
Shift: TShiftState; X, Y: Integer);
begin
if ssLeft in Shift then Spray(Image1.Canvas, x, y, 40, clRed);
end;