Here's how you set a background to an image:
self.view.backgroundColor = [UIColor colorWithPatternImage:[UIImage imageNamed:@"Background.png"]];
Edit: To write up what Felixyz said (and thanks to Manni), do this in your delegate:
window.backgroundColor = [UIColor colorWithPatternImage:[UIImage imageNamed:@"Background.png"]];
And in each view you want to have the image, do this:
self.view.backgroundColor = [UIColor clearColor];
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