Create Your Own Tiles in Windows 8 with OblyTile ? xda-developers
It is a barebones utility, but it works perfectly. You can create a tile for anything - individual files, folders, URLs or programs. You can put a label on it (or hide the label), and you specify the background PNG image to use as a tile. It asks for a small 30X30 image as well, which is only used if you zoom out the start screen. It only offers four background colors for some reason, but the color only is used when there is transparency in your PNG (like my Sims 3 tile below) or if the tile image you use is not square.
My results of customizing a few tiles:
BTW, you can use it with individual Steam games - if you right click on a game in your library, it gives you the option to create a shortcut on the desktop. If you look at the properties of the shortcut, it gives you the steam URL for the game.