The lack of being able to customize Windows 8 Start Screen is one of my critiques of the OS. I'm the kind of person that likes to make the OS feel like it's "mine". Custom skins, icons, etc..

A developer on XDA Forums has developed a small program that allows you to use your own images (found online or self made) for tiles. It has a lot of potential, and as usual is a work in progress (large tiles not supported at this version, but is coming in the future).

More info and download: [APP] OblyTile - Tile Creator for all programs. - xda-developers

Sample from CrimsonKnight13 in the forums (also a friend on [H]ardForum):