Select the field on which to sort the thumbnail view (Name, Numeric name, Type, File Size, Date, Width, Height, Orientation, Dimensions, Resolution, MD5, Similarity, Path, Unsorted, Random or User Fields). You may pick a primary and secondary sort item. To pick a primary sort item from the menu, simply select it from the menu. To pick a secondary sort item, hold the [Ctrl] key down while you select it from the menu. You may also sort in Descending order, with Thumbnail first or Thumbnail last in the view.
These selections are also available from Options | Preferences - Thumbnail View.