private void GetFiles()
{
   // Load all files in directory

   Thread t = new Thread(new ThreadStart(GetImageFiles));
   t.Priority = ThreadPriority.Lowest;
   t.Start();

}

delegate void GetImageFilesCallback();

    private void GetImageFiles()
    {
        if (this.IsHandleCreated)
        {
            if (files.InvokeRequired)
            {
                GetImageFilesCallback callback = new GetImageFilesCallback(GetImageFiles);
                this.Invoke(callback);
            }
            else
            {
                string extension = "";
                string key = "";

                foreach (string file in _files)
                {
                    extension = FileManager.GetExtension(file);
                    key = (DirectoryCurrent + file).Replace(":", "").Replace("\", "_").Replace(".", "_");

                    foreach (string knownimages in _knownImageTypes)
                    {
                        if (extension.ToLower() == knownimages)
                        {
                            foreach (ListViewItem item in files.Items)
                            {
                                if (item.Text == file)
                                {
                                    if (files.SmallImageList != null)
                                    {
                                        if (files.SmallImageList.Images[key] == null)
                                        {
                                            files.SmallImageList.Images.Add(key, Image.FromFile(DirectoryCurrent + file).GetThumbnailImage(16, 16, null, IntPtr.Zero));
                                            files.LargeImageList.Images.Add(key, Image.FromFile(DirectoryCurrent + file).GetThumbnailImage(32, 32, null, IntPtr.Zero));
                                        }

                                        files.Items[item.Index].ImageKey = key;
                                    }
                                }
                            }
                        }
                    }
                }

                files.Refresh();
            }
        }
    }

// this is your thread method
// it touches no UI elements, just loads files and passes them to the main thread
private void LoadFiles(List<string> filenames) {
   foreach (var file in filenames) {
      var key = filename.Replace(...);
      var largeBmp = Image.FromFile(...);
      var smallBmp = Image.FromFile(...);
      this.Invoke(new AddImagesDelegate(AddImages), key, largeBmp, smallBmp);
   }
}

// this executes on the main (UI) thread
private void AddImages(string key, Bitmap large, Bitmap small) {
   // add bitmaps to listview
   files.SmallImageList.Images.Add(key, small);
   files.LargeImageList.Images.Add(key, large);
}

private delegate AddImagesDelegate(string key, Bitmap large, Bitmap small);