Both shrink oversized images that bloat LCP (Largest Contentful Paint). Resizing by hand means you export each image at the right size yourself. An image CDN (a service that resizes on the fly) does it automatically per device.
Think of hand-resizing as cooking each meal to order, and an image CDN as a smart kitchen that plates the right portion every time.
When each fits:
— A small site with a handful of images: do it by hand, no extra service needed.
— A site with user uploads or thousands of images: an image CDN, because you can't resize them all manually.
Try this: pick your biggest image and check its real pixel width versus its display width in DevTools. If it's much larger, you're shipping wasted weight. One oversized hero often explains a failing score.
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Image CDN vs resizing images by hand
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