let url = NSURL(string: "http://chengkang.me/somepic.png")
if let data = NSData(contentsOfURL: url!) {
    let image = UIImage(data: data)
}

//If you want to make the code run async, you can easily achieve this with GCD:

let url = NSURL(string: image.url)
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) {
    let data = NSData(contentsOfURL: url!) //make sure your image in this url does exist, otherwise unwrap in a if let check
    dispatch_async(dispatch_get_main_queue(), { 
        imageView.image = UIImage(data: data!)
    });
}