How to crop a portion of a bitmap and save the cropped part

Dim grid(4, 4) As Point
    Dim letters() As Image
    Dim boardCoord As New Point(0, 0)
    Dim curLetter As Integer = 0
    Dim b As Image 'The image file provided
    Dim gRec(4, 4) As Rectangle
    Private Sub captureBoard()
        Dim tPoint As Point
        b = PictureBox1.Image
       'Should set the coords of the upper left corner of each square.
        For i As Integer = 0 To 3
            ' down, y
            For ii As Integer = 0 To 3 ' -> |||| x
                tPoint = New Point(boardCoord.X + (25 * ii), boardCoord.Y + (25 * i))
                grid(ii, i) = tPoint
            Next
        Next

        'Should save all the rectangles for each of the letters, to easily use when cropping.
        For x As Integer = 1 To 4
            For y As Integer = 1 To 4
                gRec(x, y) = New Rectangle(grid(x, y), New Size(20, 20))
            Next
        Next
        'Should crop each of the 16 total squares, using the preset rectangles
        For i As Integer = 1 To 16
            For x As Integer = 1 To 4
                For y As Integer = 1 To 4
                    Dim j = gRec(x, y)
                    letters(i) = CropLetters(b, j.X, j.Y, j.Size.Width, j.Size.Height)(i)
                Next
            Next
        Next
    End Sub


    Private Function CropLetters(ByVal board As Bitmap, ByVal X As Integer, ByVal Y As Integer, ByVal Width As Integer, ByVal Height As Integer) As Image()
        Dim rect As Rectangle = New Rectangle(X, Y, Width, Height) 'the current rectangle to crop
        Dim tBitmaps() As Bitmap 'the holding variable
        For i As Integer = 1 To 16 'cycles through the 16 squares and crops each one
            Dim g As Graphics = Graphics.FromImage(board)
            g.DrawImage(tBitmaps(i), 0, 0, rect, GraphicsUnit.Pixel) 'Should crop the "letter" to the holding var item
        Next
        Return tBitmaps
    End Function