为什么加载png格式图片有白边，求助！

Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
Me.FormBorderStyle = Windows.Forms.FormBorderStyle.None
Me.TransparencyKey = System.Drawing.SystemColors.Control
Me.AutoSize = True
Me.AutoSizeMode = Windows.Forms.AutoSizeMode.GrowAndShrink
PictureBox1.Image = System.Drawing.Image.FromFile(Application.StartupPath + "\picture\cs2.png")
PictureBox1.SizeMode = PictureBoxSizeMode.AutoSize
PictureBox1.ContextMenuStrip = ContextMenuStrip1
PictureBox1.Dock = DockStyle.Fill
End Sub