can't make pic box to show vb.net

Please help me.
I dont know how/why I can't make the picture box show.

VB.NET Code:

Imports System.IO
Imports System.IO.Ports
Imports System.Threading
Public Class Form1
    Dim WithEvents SerialPort1 As New IO.Ports.SerialPort

    Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
        SerialPort1.PortName = "COM4"
        SerialPort1.BaudRate = 38400
        SerialPort1.Parity = Parity.None
        SerialPort1.DataBits = 8
        SerialPort1.StopBits = StopBits.One
        SerialPort1.DtrEnable = False
        SerialPort1.ReceivedBytesThreshold = 3
        SerialPort1.Open()
    End Sub
    Dim temblin As String
    Private Sub Serial_Port1_Datareceived(ByVal sender As Object, ByVal e As EventArgs) Handles SerialPort1.DataReceived
        temblin = SerialPort1.ReadLine
        Me.Invoke(New EventHandler(AddressOf DoUpdate))
    End Sub
    Public Sub DoUpdate()
        'Lot1
        TextBox1.Text = temblin
        If temblin = "a" Then
            PictureBox1.Show()
        ElseIf temblin = "b" Then
            PictureBox1.Show()
        End If

    End Sub
End Class

Arduino code:

char Rx_data[50];
unsigned char Rx_index = 0;
int i = 0;
char msg[160];
int sig;
const int buttonPin = 2;
const int buttonPin1 = 4;
const int buttonPin2 = 6;
const int ledPin = 12;// the number of the pushbutton pin
const int ledPin1 = 13;
const int ledPin2 = 11;
const int buzzerPin = 8;
// the number of the LED pin

// variables will change:
int buttonState = 0;
int buttonState2 = 0;// variable for reading the pushbutton status
int buttonState3 = 0;
int newState = 0;

void setup() {
  Serial.begin(38400);
  // initialize the LED pin as an output:
  pinMode(ledPin, OUTPUT);   
  pinMode(ledPin1, OUTPUT);
  pinMode(ledPin2, OUTPUT);
  pinMode(buzzerPin, OUTPUT);  
  // initialize the pushbutton pin as an input:
  pinMode(buttonPin, INPUT_PULLUP); 
  pinMode(buttonPin1, INPUT_PULLUP);
  pinMode(buttonPin2, INPUT_PULLUP);  
}

void loop(){
  // read the state of the pushbutton value:
  buttonState = digitalRead(buttonPin);

  // check if the pushbutton is pressed.
  // if it is, the buttonState is HIGH:
  if (buttonState == HIGH  && buttonState2 == LOW) {     
    // turn LED on:    
    digitalWrite(ledPin, HIGH);
    SendTextMessage();
    Serial.println("a");
delay(150);  
} 
  else {
    // turn LED off:
    digitalWrite(ledPin, LOW); 
  }
    
  buttonState2 = digitalRead(buttonPin1);
  if (buttonState2 == HIGH  && buttonState3 == LOW) {     
    // turn LED on:    
    digitalWrite(ledPin1, HIGH);
    SendTextMessage();
  Serial.println("b");  
delay(150);  
} 
  else {
    // turn LED off:
   
    digitalWrite(ledPin1, LOW); 
  }
  buttonState3 = digitalRead(buttonPin2);
  if (buttonState3 == HIGH) {     
    // turn LED on:    
    digitalWrite(ledPin2, HIGH);
    digitalWrite(buzzerPin, HIGH);
    SendTextMessage();
  Serial.println("c");  
 delay(150);
  } 
  else {
    // turn LED off:
   
    digitalWrite(ledPin2, LOW); 
    digitalWrite(buzzerPin, LOW);
  }
}
//**********************************************************
void SendTextMessage()
{
 Serial.print("AT+CMGF=1\r");
 delay(100);
 Serial.println("AT + CMGS = \"09108690901\"");
 delay(100);
 Serial.println("Warning");
 delay(100);
 Serial.println((char)26);
 delay(100);
 Serial.println();
}