converting to iPad

I've recently converted my iPhone app to a universal app. Xcode created the MainWindow-iPad.xib for me automatically but I'm having trouble setting a new background for the ipad version.

In the MainViewController, I've set the following

Code:

    if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPhone) {
        background = [UIImage imageNamed: @"default.png"];
    } else {
        background = [UIImage imageNamed: @"default_ipad.png"];
    }

I've checked and it calls the default_ipad.png file but when it loads the image is shrinked to the iphone size and set at the upper left corner. Is there a way to have it call another xib file or resize it?

I'm suspecting it has something to do with the AppDelegate file where it initially sets

Code:

[window addSubview:navigationController.view]; 

Does anyone know a good tutorial on how to convert your iphone app into an ipad app?