GFork

1. Autoexpand/load and open the first picture.
   
2. Keep the thumnails open and switch picture by clicking them.
3. Add a line return before the preview so it doesn't stack at the end.
   
4. Add a thin separator line between the thumbnail grid and the preview.
   
5. Make thumbnails smaller and sort them by ratio.

Edit: Changed the default back to unsorted.
If original image order doesn't matters to you, set SORT_BY_ASPECT = true for a more compact display. (You may want to disable updates so you dont loose that edit next update.)