Make a favicon from an image

Because the same file is displayed at very different sizes: 16 pixels in a browser tab, 32 in a bookmark bar, and larger again on a pinned tile or a desktop shortcut. Handing the system one large image and letting it shrink gives a muddy 16px version, because the downscale has no idea which details mattered. Including the small sizes lets you decide instead, and the whole set is usually under 20 KB.

No. The image is scaled to fit inside the square and the canvas is grown around it, so the proportions are exactly what you started with and the extra space is transparent or white. Stretching a wide logo into a square is the most common thing favicon generators do and it is almost never what anyone wants.

Because your image is smaller than them. A 64-pixel logo blown up to 256 is a blurry square, and some launchers and operating systems will pick that largest entry to display - so adding it makes the icon look worse everywhere it is shown big. The page tells you which sizes were left out and why rather than quietly including upscaled ones.

In the root of your site, so it is served at /favicon.ico. Browsers look there by default, so no HTML change is needed for it to work. If you want to be explicit, or you are also serving larger PNG icons for mobile home screens, a link tag in the head pointing at the file does the same job.

It is the size most people actually see, so yes. Detail disappears completely at 16 pixels - a wordmark becomes a smudge and a thin outline vanishes. If your logo has a simple, high-contrast element in it, cropping to just that element before you make the icon usually produces something far more recognisable than the whole logo shrunk down.

No. Every size is rendered by your browser and the .ico container is assembled in the page, from bytes that never leave your device. A content security policy the browser enforces prevents this page from sending your image to another domain, and there is no library fetched from anywhere to do the work.