Vb.net 在visual basic中刷新picturebox图像

我正在编写一个编辑图片的程序，我有一个图片框，在这里我加载一个位图，第一个图像加载很好，但是如果我想加载第二个图像，第二个图像不会加载，第一个图像保持不变，它不会刷新。如何使用2个不同的位图更改图片框中的图像

我有PictureBox3.Image=bm，其中bm是一个位图变量，它可以很好地加载，但是如果我按其他按钮在PictureBox3.Image=bm2中加载另一个位图，它将不会加载

Private Sub Button3_Click(sender As Object, e As EventArgs) Handles Button3.Click
    Dim clr As Integer
    Dim ymax As Integer
    Dim xmax As Integer
    Dim x As Integer
    Dim y As Integer
    Dim bm As Bitmap = PictureBox1.Image
    xmax = bm.Width - 1
    ymax = bm.Height - 1
    For y = 0 To ymax
        For x = 0 To xmax
            With bm.GetPixel(x, y)
                clr = 0.21 * .R + 0.72 * .G + 0.07 * .B
            End With
            bm.SetPixel(x, y, _
            Color.FromArgb(255, clr, clr, clr))
        Next x
    Next y
    PictureBox3.Image = bm
End Sub

Private Sub Button4_Click(sender As Object, e As EventArgs) Handles Button4.Click
    Dim clr As Integer
    Dim ymax As Integer
    Dim xmax As Integer
    Dim x As Integer
    Dim y As Integer
    Dim bm2 As Bitmap = PictureBox1.Image
    xmax = bm2.Width - 1
    ymax = bm2.Height - 1
    For y = 0 To ymax
        For x = 0 To xmax
            With bm2.GetPixel(x, y)
                clr = (1 * .R + 1 * .G + 1 * .B) / 3
            End With
            bm2.SetPixel(x, y, _
            Color.FromArgb(255, clr, clr, clr))
        Next x
    Next y
    PictureBox3.Image = bm2
End Sub

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期末班

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Dim bm2 As Bitmap = new Bitmap(PictureBox1.Image)