Resize images automatically through vb.net?

Code:
Imports System.IO, System.Drawing.Imaging



Public Class Form1



    Private InputDir As String = Application.StartupPath & "\"

    Private SaveDir As String = Application.StartupPath & "\Resized Images Dir\"

    Private ResizePx As Integer = 250

    Private CalcSizeW, CalcSizeH As Integer



    Private ValidExt() As String = {".bmp", ".jpg", ".png", ".gif"}

    Private ExtArr As New List(Of String)



    Private Sub NewImage(ByVal FilePath As String, ByVal ExtType As String)



        If Not Directory.Exists(SaveDir) Then Directory.CreateDirectory(SaveDir)



        Dim bmp As New Bitmap(FilePath)



        If Width < Height Then

            CalcSizeH = ResizePx

            CalcSizeW = (bmp.Width * ResizePx) \ bmp.Height

        ElseIf Width > Height Then

            CalcSizeW = ResizePx

            CalcSizeH = (bmp.Height * ResizePx) \ bmp.Width

        Else

            'it's a square image so put both dimensions to ResizePx

            Width = Height = ResizePx

        End If



        Dim ResizeImg As New Bitmap(CalcSizeW, CalcSizeH)

        Dim g As Graphics = Graphics.FromImage(ResizeImg)



        With g

            .InterpolationMode = Drawing2D.InterpolationMode.HighQualityBicubic

            .DrawImage(bmp, New Rectangle(0, 0, Width, Height), New Rectangle(0, 0, bmp.Width, bmp.Height), GraphicsUnit.Pixel)

            .Dispose()

        End With



        bmp.Dispose()



        Dim FName As String = SaveDir & "Resized 1" & ExtType



        Dim Num As Integer = 1

        While File.Exists(FName)

            Num += 1

            FName = SaveDir & "Resized " & Num & ExtType

        End While



        Select Case ExtType.ToLower

            Case Is = ".jpg"

                ResizeImg.Save(FName, ImageFormat.Jpeg)

            Case Is = ".png"

                ResizeImg.Save(FName, ImageFormat.Png)

            Case Is = ".bmp"

                ResizeImg.Save(FName, ImageFormat.Bmp)

            Case Is = ".gif"

                ResizeImg.Save(FName, ImageFormat.Gif)

        End Select



        ResizeImg.Dispose()



    End Sub



    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        ExtArr.AddRange(ValidExt)

        For Each File As String In Directory.GetFiles(InputDir)

            Dim FileExt As String = File.Substring(File.LastIndexOf("."), File.Length - File.LastIndexOf(".")).ToLower

            If ExtArr.Contains(FileExt) Then

                NewImage(File, FileExt)

            End If

        Next

        MessageBox.Show("Done resizing images", "Action completed", MessageBoxButtons.OK, MessageBoxIcon.Asterisk)

    End Sub

End Class