How does Safari name the 'Apple Touch Icon' files?
If you bookmark a page that shows an icon for it under the Bookmarks menu, it (the image) is stored in the ~/Library/Safari/WebpageIcons.db file, which is an SQLite version 3 database. Images stored within the database can be in different formats, e.g. .ico, .png, etc.

If you really want to change the icon, that's in the database, I've done it with DB Browser for SQLite.