程序关键部分简化如下：
Public Class Form1
    Dim a As PictureBox = PictureBox1

    Private Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
        a.Image = Image.FromFile(Application.StartupPath & "\1.jpg")
    End Sub
End Class
结果显示“未将对象引用设置到对象的实例”，图片不能导入。
为什么会出现这种情况？
如何解决？（保留全局变量的声明）

Public Class Form1
 Public a As New PictureBox  '定义公共变量
    Private Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles Button1.Click
        a = Me.PictureBox1
        a.Image = Image.FromFile(Application.StartupPath & "\1.jpg")
    End Sub
End Class