You're checking in 200 product photos. Should they be JPG? PNG? WebP? AVIF? Whatever the designer dragged in? This is the question every developer ducks until performance reviews force the conversation. The honest answer is "it depends" — but the dependencies are concrete and learnable, and once you know them you stop wasting bandwidth on the wrong format. This is a practical guide to the five formats you'll actually touch (JPG, PNG, WebP, AVIF, HEIC), plus a quick note on SVG and GIF. File sizes, when each wins, browser support, the conversion gotchas. The decision tree Before the deep dive, here's the rough mental model: You have... Use...…