I use TGP Gallery Maker,
http://tgpgallerysubmitter.com/. It will make the thumbs for you. I didn't really care for their templates so made some changes and just pasted them in after I've made a gallery.
If I do need to make a bunch of thumbs, I use Image Resizer Pro,
http://www.download.com/Image-Resize...ml?tag=lst-0-1. You can buy it if you want but the free version lets you resize five at a time.