After installing, your shop's images will be added to the queue and optimized the first time. 

Usually it takes hours depends on how many images do you have in your shop. The maximum time is 10 hours .

You can see estimate processing time in Image Optimization page.

New Images: every time you upload images to products, you should go to the app and process the optimization  manually. Any optimized image will not enter the flow again.

When finish, it will show message like this: