Save drawn Image to file

I want my programm to save an Text as Image to a file. This is what I could do so far:

Public Sub StringToPrint()
    Dim formGraphics As Graphics = PictureBox1.CreateGraphics() 'draw text to PictureBox1
    Dim drawString As String = TextBox1.Text
    Dim drawFont As System.Drawing.Font = New System.Drawing.Font("Arial", 16)
    Dim drawBrush As System.Drawing.SolidBrush = New System.Drawing.SolidBrush(System.Drawing.Color.Black)
    Dim drawFormat As System.Drawing.StringFormat = New System.Drawing.StringFormat()
    formGraphics.DrawString(drawString, drawFont, drawBrush, New PointF(0, 0), drawFormat)

    formGraphics.SmoothingMode = Drawing2D.SmoothingMode.HighQuality
    formGraphics.TextRenderingHint = Drawing.Text.TextRenderingHint.ClearTypeGridFit
    formGraphics.CompositingQuality = Drawing2D.CompositingQuality.HighQuality

    ' Save the screenshot  This part right here is not working...
    'SaveFileDialog1.Filter = "JPEG Files (*.jpeg*)|*.jpeg"
    'If SaveFileDialog1.ShowDialog = Windows.Forms.DialogResult.OK _
    ' Then
    '    'PictureBox1.Image.Save(SaveFileDialog1.FileName, ImageFormat.Jpeg)
    'End If 'It says PcitureBox1 is Nothing.

End Sub

I only want to save the Text in the PictureBox1 as a transparent image file.