Image optimization covers everything that makes an image load fast and communicate clearly without needing more file weight than the job requires: correct file format (WebP or AVIF over older JPEG or PNG in most cases), compression that trims size without a visible quality hit, dimensions actually matched to where the image displays rather than a much larger original scaled down by CSS, and descriptive alt text and filenames.
Unoptimized images are one of the most common reasons a page fails Core Web Vitals, since images are frequently the single heaviest asset on a page and directly affect metrics like Largest Contentful Paint. Beyond speed, optimized images also carry more SEO value: a descriptive filename and alt text give search engines real signal about what the image shows, which matters for image search specifically and adds a small amount of relevant context to the page as a whole. Lazy loading, deferring images below the fold until a visitor scrolls near them, rounds out the practice by keeping the initial page load focused on what's actually visible first.
