iOS: By default, you can't change the look of icons in iOS without jailbreaking. Iconical is a new app that makes the process possible with a clever workaround that creates shortcuts to apps using any icon you want.
Creating icons is easy. Just select the app you want to change, and then you can pick an icon from a URL, draw your own, or even take a picture. Iconical then changes that into a webapp so you can launch it from your home screen. This means you have two icons for an app sitting around, but you can hide the default away in a folder if you like.
Iconical effectively uses the same tricks that other apps such as Drafts or Fantastical use to communicate with each other, but does so to change the look of an icon. It's not a perfect solution, but it gets the job done.