Clicking on image does not enlarge image: Why and how

This is a frequent question. Why is it when I click on an image (photo, screen shot, etc.), it does not enlarge and how do I make it enlarge?I was told that if you upload a photo or image in the Blogger post editor window COMPOSE mode, and you drag and drop the photo or image, it will result in exactly the above problem. I myself practically never use the COMPOSE mode, always the EDIT HTML mode