A thread fails to update the image as expected - multithreading

Question

I am trying to write a Delphi code to update the TImage components on a from. I was using C++ for long time but don't have much experience in Delphi;however, I need to use Delphi for some reason. I obtained and modified a similar code to do the task as shown as follows. I have 4 TImages components placed on a form, I start a thread to update the images when the player press the button. I have 6 resource bitmaps with numbered in 1 to 6 embedded in the project and the code will randomly pick and load the resource bitmap into the TImage. I find that using the thread sometimes, some images will not be drawn (just blank like no image being loaded). I know you may be wondering why I need to use a thread and why not just call ShowRanodmImage when the button is pressed. In this simple case, surely I don't have to use thread instead but I am going to apply to some situation that threading must be used. So my question is why the following code will occationally not show the proper image? How to fix that? Thanks a lot.
unit TTUnit;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms, Dialogs, StdCtrls, ExtCtrls;
type
TTestThread = class;
TTestForm = class(TForm)
NextBTN: TButton;
P11: TPanel;
P12: TPanel;
P13: TPanel;
P14: TPanel;
CELL11: TImage;
CELL12: TImage;
CELL22: TImage;
CELL21: TImage;
procedure FormCreate(Sender: TObject);
procedure NextBTNClick(Sender: TObject);
private
myThread :TTestThread;
pImgs: array[1..2, 1..2] of TImage;
public
procedure ShowRandomImg;
end;
TTestThread = class(TThread)
private
myForm :TTestForm;
protected
procedure Execute(); override;
public
constructor Create(aFrom :TTestForm; CreateSuspended: Boolean);
end;
var
TestForm: TTestForm;
implementation
{$R *.dfm}
procedure TTestForm.FormCreate(Sender: TObject);
begin
myThread := TTestThread.Create(Self, True);
pImgs[1][1]:=CELL11; pImgs[1][2]:=CELL12;
pImgs[2][1]:=CELL21; pImgs[2][2]:=CELL22;
end;
procedure TTestForm.NextBTNClick(Sender: TObject);
begin
myThread.Resume;
end;
procedure TTestForm.ShowRandomImg;
var
i, j :Integer;
r :array[1..2, 1..2] of Integer;
begin
Self.NextBTN.Enabled := false;
r[1][1] := Random(6)+1;
r[1][2] := Random(6)+1;
r[2][1] := Random(6)+1;
r[2][2] := Random(6)+1;
for i := 1 to 2 do
begin
for j := 1 to 2 do
begin
pImgs[i][j].Picture.Bitmap.LoadFromResourceID(HInstance, r[i][j]);
pImgs[i][j].Canvas.Font.Size := 38;
pImgs[i][j].Canvas.Font.Color := clRed;
pImgs[i][j].Canvas.Font.Style := [fsBold];
pImgs[i][j].Canvas.TextOut(20, 20, inttostr(r[i][j]));
end;
end;
Self.NextBTN.Enabled := true;
end;
////////////////////
constructor TTestThread.Create(aFrom: TTestForm; CreateSuspended: Boolean);
begin
inherited Create(CreateSuspended);
Self.myForm := aFrom;
end;
procedure TTestThread.Execute;
begin
inherited;
Repeat
Self.myForm.ShowRandomImg;
Self.Suspend;
Until Terminated;
end;
end.