Ask HN: What are best ways to down res JPG images?

Our cloud service has a ton of jpgs loaded by users. The images are loaded, then viewed occasionally by auditors and sometimes automated OCR. So written once and read/viewed ~4 times. We want to look at ways to save storage footprint, and I'd like to down sample the JPGs yet keep enough quality to keep users and automated OCR systems happy.

I'm hoping that someone has studied this problem and any pointers would be greatly appreciated.