speeding up images by reducing resolution

We have a WordPress website where a whole bunch of images already uploaded are way too high resolution, causing the site to be slow. I've tied Smush Pro but I don't think it adresses the resolution problem although it is very effective at squeezing out more room.

What is the best and easiest tool and/or plug in to autogenerate multiple image sizes within the WordPress site so the resolution and image sizes are optimal, and the site loads as fast as possible?

Many thanks for you help!