How would you explain image optimization in seo to a client?

I keep it to two points. Big images make your pages slow, slow pages lose visitors, so we shrink them without them looking any worse. And Google can't see a picture, so we describe it in words (the file name and alt text) which also helps people using screen readers and gets you into image search. Then I pull up one of their pages where the hero image is 4MB. That usually ends the conversation.