I need to fit an image as background in my swift 3 application. I set the background with this:
self.view.backgroundColor = UIColor(patternImage: UIImage(named: "bg_sunrise")!)
It depends on what you really want.
If you need "aspect fit", you need to either drop your image into a UIImageView like emresancaktar provided. The issue with this - and by your use of the view's backgroundColor you suggest this - is that you wish to put things on top of said UIImageView. This is complicating things.
If you just need to scale to the view size with no regard to "aspect fit", you can use it's layer:
view.layer.contents = UIImage(named: "bg_sunrise")?.cgImage
Finally, if your like using a view's layer but do need "aspect fit", you'll need to work with a CGImage context, possibly using the UIImage.drawInRect() method after making some calculations. A quick search on "CALayer draw image aspect fit" yielded this gist. It's a little old but there's a Swift version there. You can cobble what you need to return a CALayer instead of a UIImage.