UIBezierPathで角丸にする ref: http://qiita.com/morizotter/items/8f5032e41339f1d82fa9

file0.swift

import UIKit

class RoundedCornersView: UIView {

    override func drawRect(rect: CGRect) {
        UIBezierPath(roundedRect: self.bounds, cornerRadius: CGRectGetHeight(self.bounds) / 2).addClip()
        UIImage(named: "rocket")?.drawInRect(self.bounds)
    }
}

file1.swift

import UIKit

class LayerRoundedCornersView: UIView {

    override func drawRect(rect: CGRect) {
        UIImage(named: "rocket")?.drawInRect(self.bounds)

        let maskPath = UIBezierPath(roundedRect: self.bounds, cornerRadius: CGRectGetWidth(self.bounds) / 2)
        let maskLayer = CAShapeLayer()
        maskLayer.path = maskPath.CGPath
        self.layer.mask = maskLayer;
    }
}