【Swift】UIImageをリサイズする ref: http://qiita.com/ryokosuge/items/d997389529faffab33ba

UIImageExtension.swift

extension UIImageExtension {

    func resize(size: CGSize) -> UIImage {
        let widthRatio = size.width / self.size.width
        let heightRatio = size.height / self.size.height
        let ratio = (widthRatio < heightRatio) ? widthRatio : heightRatio
        let resizedSize = CGSize(width: (self.size.width * ratio), height: (self.size.height * ratio))
        UIGraphicsBeginImageContextWithOptions(resizedSize, false, 2)
        drawInRect(CGRect(x: 0, y: 0, width: resizedSize.width, height: resizedSize.height))
        let resizedImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return resizedImage
    }

    // 比率だけ指定する場合
    func resize(#ratio: CGFloat) -> UIImage {
        let resizedSize = CGSize(width: Int(self.size.width * ratio), height: Int(self.size.height * ratio))
        UIGraphicsBeginImageContextWithOptions(resizedSize, false, 2)
        drawInRect(CGRect(x: 0, y: 0, width: resizedSize.width, height: resizedSize.height))
        let resizedImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return resizedImage
    }
}