Will the optimized images automatically update if they are already posted on my site?

Hi,

Yes it is, images are automatically replaced by the optimized ones.
You just need to wait a bit during the optimization process and everything is done

Cheers,

Hi,

Yes it will optimize by default all images in the wp-content/uploads and wp-content/themes folders.
But you can also change it in the settings if you need to optimize other folders

Best regards