Michael, if all you want to do is optimize already uploaded images then I recommend you look at the free tiers of EWWW Image Optimizer (https://wordpress.org/plugins/ewww-image-optimizer/) and Optimus (https://wordpress.org/plugins/optimus/).

Both of these should stand you in good stead. I use EWWW and Optimus’ manual mode to deal with the images on my image heavy site.