What's a good program to batch resize 100,000 jpg images?
I'm a little surprised that no one has mentioned the easiest, cheapest, and least technical option:
Automator
First, open Automator, which is in your Applications folder.
Next, choose to create a new workflow:

Next, add the following steps to the workflow by dragging and dropping:

When you run this, Automator is going to:

pop up a window asking you to pick some files (as many as you want)
pop up another window asking where you want to save the resized copies
ask you how big you want the resized copies to be.  You'll be able to choose either a fixed size or a percentage.  All the images will be resized to either that size or by that percentage.

(The &quot;show this action when the workflow runs&quot; checkbox means that the workflow is going to pause, display the option, and allow you to change it then)
Once you do that, Automator is going to churn and burn, and when it's done it'll beep at you.
Unfortunately, it doesn't seem to have an option to choose JPEG quality.  However, for the price (free), it's pretty dang convenient.