There are two essential components of optimizing images for SEO. One would deal with including the needed keywords in image titles, alt text, URLs, etc. Those you consider for every image on your page. Another component is image performance.
One of Google's ranking factors is Performance Score (source ).
You can predict an improvement in Google Performance Score, user engagement, and the subsequent drop in page load time and bounced visitors. Here is a prediction model .
Once you're done picking the priority images for optimization, here's a guide on how you do it with Image CDN.