Upload a square logo or image and get back a proper favicon.ico containing 16, 32, 48, 64, 128, and 256px versions — the same way professional sites build their favicon.
Upload the resulting favicon.ico to the root folder of your website, then add this to your HTML <head>: <link rel="icon" href="/favicon.ico">. Most browsers will also pick it up automatically from your site root without any HTML tag at all.