Originally Posted by The Porn Nerd

The best one for me, and the fastest, is called Faststone Photo Resizer. They have a whole suite of programs but the resizer will do what you want:

https://www.faststone.org/FSResizerDetail.htm

Also free and done in batch.

Originally Posted by CurrentlySober

GIMP, but you'll need a plugin for it: 'BIMP'