What you get
- favicon.ico holding the sizes you tick (16, 32, 48 and 64 to begin with; 128 and 256 if you want them). Browsers pick 16 or 32 for the tab, Windows uses 48 and larger, and Google asks for a favicon of at least 48 × 48 pixels for its search results.
- In the ZIP pack, a PNG of each size, a 180-pixel apple-touch-icon.png for iPhone home screens, the 192 and 512 pixel icons Android and web app manifests use, and a text file with the lines to paste into your website’s <head>.
Each size inside the .ico is stored as a PNG, which every current browser and Windows since Vista read, and which keeps full transparency. Windows XP and older cannot show PNG-based icons.
Getting a clear icon at 16 pixels
A favicon is tiny. A full logo with a name rarely reads at 16 × 16; the symbol alone, or a single bold letter, does. Start from a square picture at least 256 pixels across, or an SVG, which is drawn afresh at each size so thin lines stay crisp. A picture that is not square can be shown whole with empty space around it, or cropped to fill the square.
Transparent backgrounds are kept. The Apple icon always gets a background (white unless you choose one), because iPhones fill a see-through home-screen icon with black.