cropping and stretching

[aamir55]

i want to crop the picture but when i draw a rectangle on the picture and releases the mouse,picture is croped but picture is stretched
and picture quality becomes bad.
i m worry due this problem...i can't understand it
i m using a picture box and a panel
i m attaching the code along database files...bcoz picture are stored in database

Private Sub pb_MouseDown(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles pb.MouseDown
        Try
            If e.Button = Windows.Forms.MouseButtons.Left Then
                cropX = e.X
                cropY = e.Y
                cropPen = New Pen(cropPenColor, cropPenSize)
                cropPen.DashStyle = Drawing2D.DashStyle.DashDotDot
                Cursor = Cursors.Cross

            End If
            pb.Refresh()
        Catch ex As Exception

        End Try
    End Sub

    Private Sub pb_MouseMove(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles pb.MouseMove
        Try
            If pb.Image Is Nothing Then Exit Sub
            If e.Button = Windows.Forms.MouseButtons.Left Then
                pb.Refresh()
                cropWidth = e.X - cropX
                cropHeight = e.Y - cropY
                pb.CreateGraphics.DrawRectangle(cropPen, cropX, cropY, cropWidth, cropHeight)

            End If
        Catch ex As Exception

        End Try
    End Sub

    Private Sub pb_MouseUp(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles pb.MouseUp
        Try
            Cursor = Cursors.Default
            Try
                If cropWidth < 1 Then
                    Exit Sub

                End If
                Dim rect As Rectangle = New Rectangle(cropX, cropY, cropWidth, cropHeight)
                Dim bit As Bitmap = New Bitmap(pb.Image, pb.Width, pb.Height)
                cropBitmap = New Bitmap(cropWidth, cropHeight)
                Dim g As Graphics = Graphics.FromImage(cropBitmap)

                g.InterpolationMode = Drawing2D.InterpolationMode.HighQualityBicubic
                g.PixelOffsetMode = Drawing2D.PixelOffsetMode.HighQuality
                g.CompositingQuality = Drawing2D.CompositingQuality.HighQuality
                g.DrawImage(bit, 0, 0, rect, GraphicsUnit.Pixel)
                pb.Image = cropBitmap


            Catch ex As Exception
            End Try
        Catch ex As Exception

        End Try
    End Sub

kindly help me........thanks