Batch Image Processor

Hundreds. Each one is decoded, drawn and re-encoded in turn rather than all at once, and the tab is given a moment between them so the progress line keeps moving and the page stays answerable. What you will run into is memory rather than a fixed limit: very large photographs at full resolution add up, and a few hundred of them will make a browser struggle. Resizing as part of the batch makes that far better, because the big version never has to exist twice.

They are HEIC or TIFF. No browser can decode either — the HEIC decoder is patent-encumbered and no engine ships one — so this is the browser's limit rather than the page's. They are recognised from the file's own bytes and set aside before the run starts, so you find out immediately rather than after waiting for everything else. On an iPhone, Settings → Camera → Formats → Most Compatible switches new photos to JPEG.

It builds each new file name. {name} is the original without its extension, {n} is a number padded to the size of the batch so a hundred files sort correctly, and {w}, {h} and {wh} are the size after resizing rather than before. The extension is never yours to set — it always matches the format the file actually is, because a PNG called .jpg opens in a browser and is refused by half the programs you try next.

They cannot. Inside a ZIP a repeated name silently replaces the earlier file, so half a batch would arrive missing with nothing to show why. Duplicates get a numbered suffix instead — which matters most with a fixed pattern that has no {n} in it, the commonest way to lose a folder.

WebP for almost everything on the web: it is smaller than JPEG at the same quality and every current browser reads it. AVIF is smaller still but slower to encode, which is noticeable across hundreds of files. JPEG when something old has to open it. PNG only when you need transparency or an exact copy of every pixel — quality does not apply to it, so the slider disappears.

No. The images are decoded in the page, redrawn on a canvas, re-encoded by the browser and packed into the archive there. Nothing is sent anywhere, which is also why it works with no connection at all.