Will it blow up photos that are already small?

No, as long as "Never enlarge a smaller image" stays on, which it is by default. When a resize would make a picture bigger than it started, the batch leaves its pixel dimensions alone. A 400px thumbnail asked to become 2000px wide simply stays 400px wide.

What do the rename tokens mean?

Five tokens are supported: {name} is the original file name without its extension, {n} is the file's position in the batch starting at 1, {n:3} is that same number zero-padded to three digits, {ext} is the extension the output will really carry, and {date} is today's date as YYYY-MM-DD. Anything else in braces is left exactly as you typed it, so a typo shows up in the preview instead of vanishing.

Can the batch processor write AVIF?

No. Save as offers Keep original, JPG, PNG and WebP only. If you specifically want AVIF, use Convert Image Format on a single file, which offers AVIF when your browser can encode it.

What happens to iPhone HEIC photos?

A browser can decode more formats than it can write. If you leave Save as on Keep original and the recipe touches the pixels, a HEIC source comes out as PNG, which is usually far larger than the original. Set Save as to WebP or JPG explicitly for phone photos.

Are the photos uploaded anywhere?

No. Every decode, resize, re-encode and the zip itself happen in the browser tab. There is no account, no sync and no server round trip, and closing the tab is all it takes to be rid of the files.

Batch-Resize a Folder of Photos for Your Website

Resize, convert and rename a whole folder of photos in one pass, then download the lot as a single zip. Runs in your browser, so nothing is uploaded.