How Are Favicon's Used?
A favicon, also known as a shortcut icon, website icon, tab icon, url icon, or bookmark icon is a file containing one of more small icons associated with a particular website. Search Engines and popular browsers will display the icon as such.
If links for both PNG and ICO favicons are present, Firefox and Safari will use the icon that appears last in the code. Chrome for MAC will use whichever icon is formatted as an ICO, otherwise it will use the 32x32 icon. Chrome for Windows will use either the first icon or the one that is 16x16 - otherwise it will then use the ICO file. If none of these icons are available, both Chromes will use whichever icon comes first - the exact opposite of what Firefox and Safari use. Chrome for Mac will always ignore the 16x16 icon and use the 32x32 icon, but will reduce it down to 16x16 on non-retina displays. Opera will choose icons at random.
Googlebot will choose the largest provided size from your provided < link rel="icon" >, < link rel="apple-touch-icon" > or < link rel="shortcut icon" >. Googlebot prefers a .png or .jpg over a .ico file. If nothing is available, Googlebot will default back to whatever file is hosted at /favicon.ico