You can manually create thumbnails... the entire directory at once.
Download the Image Resizer PowerToy for your O.S. (there are different flavors for XP, Vista, 7, etc).
Copy all of the images to your thumbnails directory.
Do a Control-A (Select All)
Right-click one of the images.
Select "Resize Images"
Make them the size that you want.
That's how I did all 2900+ of my images that I imported, without going through uploading each one and having it run the thumbnail routine.