I can´t upload images with the RestAPI in Swift

ios
rest

#1

I have coded this:
@IBAction func tweet(sender: AnyObject) {

        let url = NSURL(string: "https://api.twitter.com/1.1/statuses/update.json")

        if let txt = textView.text, let img = imageView.image {
            let request = SLRequest(forServiceType: SLServiceTypeTwitter, requestMethod: .POST, URL: url, parameters: ["status": txt, "media_id_string": img,])
                request.account = twitterAccount
        
            request.performRequestWithHandler { (data, response, error) -> Void in
                if response.statusCode != 200 {
                print(error)
            }
            }
        } else {
            print("textToTweet or imageToTweet is nil")
        }
   
    }`

It´s only tweeting the text.
Thank you!