If an icon is not explicitly provided one will be generated, the results can be a little hit and miss.

Providing an asset involves creating a 72 x 72 pixels png file, saving it as apple-touch-icon.png and saving it into the root of your web folder.

The rounded corners and highlight effect will be generated automatically, you do not need to create them in the png.

The icon I have used for this site (as seen in the top image) is shown here.