public IReadOnlyCollection<int> GetSelectedThumbnailPageIndexes()

Public Function GetSelectedThumbnailPageIndexes As IReadOnlyCollection(Of Integer)

using System.Windows.Media.Imaging;
using System.IO;
// ...
private void simpleButton_Click(object sender, RoutedEventArgs e) {
  // Obtains the selected page indexes.
  var pages = viewer.ActualThumbnailsViewerSettings.GetSelectedThumbnailPageIndexes();
  // Saves each page from the collection to an image.
  foreach (var i in pages) {
    BitmapSource image = viewer.CreateBitmap(i, 1000);
    PngBitmapEncoder encoder = new PngBitmapEncoder();
    encoder.Frames.Add(BitmapFrame.Create(image));
    using (var fileStream = new FileStream($"..\\MyBitmap{i + 1}.bmp", FileMode.Create)) {
        encoder.Save(fileStream);
    }
  }
}

Imports System.Windows.Media.Imaging
Imports System.IO
' ...
Private Sub simpleButton_Click(ByVal sender As Object, ByVal e As RoutedEventArgs)
  ' Obtains the selected page indexes.
  Dim pages = viewer.ActualThumbnailsViewerSettings.GetSelectedThumbnailPageIndexes()
  ' Saves each page from the collection to an image.
  For Each i In pages
    Dim image As BitmapSource = viewer.CreateBitmap(i, 1000)
    Dim encoder As New PngBitmapEncoder()
    encoder.Frames.Add(BitmapFrame.Create(image))
    Using fileStream = New FileStream($"..\MyBitmap{i + 1}.bmp", FileMode.Create)
        encoder.Save(fileStream)
    End Using
  Next i
End Sub