I have a Mac, and I use a little shareware program 
called QuickNailer. To make a long story short, I 
simply point it at a directory of .jpgs (it takes 
many formats), and it creates a set of thumbnails 
from them.

There are lots of "nerd knobs" about size, quality, 
where to put the thumbnails, etc.

If you're not on a Mac, you might want to check the 
shareware sites to see if there's something 
similar, if not a version of this.