The graphic is most commonly an .ico file (similar to a .bmp) that measures 16 x 16 pixels. Other files, such as .png’s, .gif’s, or .jpg’s can be used and are properly read by many modern browsers, however .ico is the most universal file format. It can also be called something other than favicon.ico as long as you link to it property in your head tag. It should be mentioned that Internet Explorer and other browsers can read and display 32 x 32 pixel formats, but most places where you’d expect to see a favicon will still shrink it (downsample it) to show up as 16 x 16 px.
I would recommend using favicon.ico (name it favicon and use extension .ico) at 16×16 because it’s the most widely accepted and recognized format.
How do you create a favicon?
The easiest way is to create a graphic in a square format using your favorite image editor, save the file as a png, jpg, or gif, and then run it through this tool…
…then save the file to your root directory in your webserver. Be sure to add a reference to it in between your head tags (as specified in the instructions on the home page). It’s that simple.