Method SetUserTile
SetUserTile(string, int, string)
Sets the user tile. I found it with reverse engineering. According to http://msdn.microsoft.com/en-us/library/bb776892.aspx, it's new to Windows 7 and 2008 R2, I tested and it works. I also found that it doesn't work on XP.
public static extern void SetUserTile(string username, int whatever, string picpath)