[AtPeterG]

Das geht viel einfacher:

Public Class Form1

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    Select Case picturebox.image
        Case Picturebox1.Image = My.Resources._1
            PictureBox1.Image = My.Resources._2
        Case else
            PictureBox1.Image = My.Resources._1
        End Select
End Sub

Das geht auch mit einer "if then" Abfrage.

Public Class Form1

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    If Picturebox1.Image = My.Resources._1 then
            PictureBox1.Image = My.Resources._2
        else
            PictureBox1.Image = My.Resources._1
        End if
End Sub

Vorher natürlich die Bilder in die Resouces des Projektes laden.