Try adding the free AmpProject.org plugin because it is now quite refined, stores your original image on Google’s free CDN and supplies an optimised image version depending on the browser’s viewport.
No need to optimise images, use their App for that 
This works seamlessly not only for mobiles but also for desktops. This feature saves a tremendous amount of coding and I especially like the built-in feature that instantly renders images only for the available viewport, remaining images appear only when the web page is scrolled!
An excellent plugin and well with the effort to learn their strict requirements.
Edit:
I forgot to mention Google gives free mobile SEO preference to web pages but only if they confirm to their strict validation requirements. Some W3.org validation specifications are ignored.