On iOS devices using the Safari browser when you add a web page shortcut to your home screen if will display as a screenshot of the website by default. To use an icon instead do the following:

Create your icon as a 180x180px .png file (good for all iOS devices – those with regular displays and retina displays too)

Name it: “apple-touch-icon.png”

Upload it to your web site.

Add the following to the head section of your web pages (the href path can be to wherever you have stored the icon):

<link rel="apple-touch-icon" href="/myimages/apple-touch-icon.png">

This will also be used by Android devices as a fallback, so is good for both.