The article you reference is for enabling wide tiles on Windows Phone 8 devices from within a Windows Phone 7.1 project. Your 7.1 app can still install on a WP8. If your app has been upgraded to a Windows Phone 8 project then use the Flip Tile Template to set the WideBackContent and WideBackground Images.
FlipTileData TileData = new FlipTileData()
{
Title = "[title]",
BackTitle = "[back of Tile title]",
BackContent = "[back of medium Tile size content]",
WideBackContent = "[back of wide Tile size content]",
Count = [count],
SmallBackgroundImage = [small Tile size URI],
BackgroundImage = [front of medium Tile size URI],
BackBackgroundImage = [back of medium Tile size URI],
WideBackgroundImage = [front of wide Tile size URI],
WideBackBackgroundImage = [back of wide Tile size URI],
};
Otherwise, if you really do have a 7.1 project, I recommend the Mangopollo project to simplify the process. There is a //Build/ session by Thomas Fennel that may be helpful.