Image Source to file

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VB

WPF

Hi guys, I presume this is a no-brainer for WPF coders?
Below is a function that returns an image source, but what I need to do is create a new jpg file, simple question is how do I convert a bitmap image to a file on disk? below is a basic func that I use to view thumbnails so I'm half way there.
any ideas would be helpful thanks.

Public Function ScaleImageToFit(ByVal sourceImg As String, ByVal pbWidth As Integer, ByVal pbHeight As Integer) As BitmapImage

        Dim sourceImage As New BitmapImage()
        sourceImage.BeginInit()
        sourceImage.UriSource = New Uri(sourceImg, UriKind.Relative)
        sourceImage.EndInit()

        Dim scale_factor As Single = 0

        ' determine scale_factor based on source image size.
        Dim srcWidth As Integer = sourceImage.Width
        Dim srcHeight As Integer = sourceImage.Height
        Dim myImage As New Image

        If srcWidth > srcHeight Then  ' landscape
            scale_factor = pbWidth / srcWidth
            myImage.Width = pbWidth * scale_factor
            sourceImage.DecodePixelWidth = myImage.Width
        Else    ' portrait

            scale_factor = pbHeight / srcHeight  ' make it fit to pb height
            myImage.Height = pbWidth * scale_factor
            sourceImage.DecodePixelHeight = myImage.Height
        End If

        myImage.Source = sourceImage

        Return myImage.Source

    End Function