PromiseKit.m

dispatch_promise(^{
    return md5(email);
}).then(^(NSString *md5){
    return [NSURLConnection GET:@"http://gravatar.com/%@", md5];
}).then(^(UIImage *gravatarImage){
    self.imageView.image = gravatarImage;
});

PromiseKit.swift

dispatch_promise{ () -> String in
    md5(email)
}.then{ (md5:String) -> NSURLConnection in
    NSURLConnection.GET("http://gravatar.com/\(md5)")
}.then{ [weak self] (gravatarImage:UIImage) in
    self?.imageView.image = gravatarImage
};