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">