Hmm, so how did the Goog make a tile icon?!
So next, I spoofed my phone's browser and curled Google's home page to see what it showed - maybe there was some API or something you could call or a meta-tag that the rest of us don't know about. There isn't - it's actually a lot simpler than that. If you look at the screen captures above, you'll see that WPCentral showed the steps you have to take to add an icon: You click the button on the home page, then you see a blue screen with a logo and instructions to then use the menu to pin a tile.
The key is that Google is using an SVG image file on that page!
More details, and the vital code snippet for web developers are over on Beattie's site.