从iOS到Facebook发布到墙上显示空白图像

Question

我尝试使用以下代码发布带有iOS应用中图片的留言信息：

- (void) putOnFacebookWall:(UIImage *) screenShot{

    NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];

    NSString *filePath = [NSTemporaryDirectory() stringByAppendingPathComponent:@"temp.png"];

    [UIImagePNGRepresentation(screenShot) writeToFile:filePath options:NSAtomicWrite error:nil];

    NSData *imageData = UIImagePNGRepresentation([UIImage imageNamed:@"ironman.jpg"]);

    NSURL *url = [NSURL URLWithString:@"https://graph.facebook.com/me/feed"];
    ASIFormDataRequest *request = [ASIFormDataRequest requestWithURL:url];

    //
    [request setPostValue:imageData forKey:@"picture"];

    [request setPostValue:@"http://apps.facebook.com/mobie-hair-wizz" forKey:@"link"];

    [request setPostValue:@"test" forKey:@"message"];

    [request setPostValue:[defaults objectForKey:@"FBAccessTokenKey"] forKey:@"access_token"];
    [request setDidFinishSelector:@selector(sendToPhotosFinished:)];

    [request setDelegate:self];
    [request startAsynchronous];

}

我得到的是：

https://www.dropbox.com/s/z5nzm271zkadq4j/wall.png?m

图像不显示的原因是什么？

我在FB的iOS代码中也得到了积极的回应，说上传成功了。