ThumbnailViewer allows to select and deselect thumbnails programmatically.

All thumbnails in ThumbnailViewer can be selected using the following code: thumbnailViewer1.SelectedIndices.AddRange(0, thumbnailViewer1.Images.Count - 1);

All thumbnails in ThumbnailViewer can be deselected using the following code: thumbnailViewer1.SelectedIndices.Clear();