Multiple Images for Multiple Devices
Favicons are an important but sadly often overlooked part of your website. These icons act as your branding when your users save your site to their desktop, tab or bar.
The reason for the multitude of favicons you need is down to history. The fundamental idea being that different operating systems need different icon types and sizes to showcase your favicon in it’s optimal resolution.
Some mobile OS’s such as iOS will require ‘Retina’ (high resolution) versions of your favicon to ensure it looks great. This is both positive and negative, positive being your users get to see an awesome sharp graphic, negative being it’s another favicon you need to generate.
Automating the Process
There are several great services out there that help automate the process of manually resizing your favicons. The Real Favicon Generator for example lets you configure several options to tailor your icon to all the OS’s you’ll need (such as Android, iOS and Windows)
If you’re interested in learning more you can read the follow SitePoint article that illustrates this in further detail.