Downloading JSON image Problem

#import <UIKit/UIKit.h>

@interface MasterViewController : UITableViewController

@property (strong, nonatomic) NSArray *tajuk;

@end

#import "MasterViewController.h"

#import "DetailViewController.h"

@interface MasterViewController () {
    NSMutableArray *_objects;
}
@end

@implementation MasterViewController

- (void)awakeFromNib
{
    [super awakeFromNib];
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    //self.tajuk = [NSArray arrayWithObjects:@"barca",@"kelate",@"jdt", nil];

    NSURL *url = [NSURL URLWithString:@"http://blog.teamtreehouse.com/api/get_recent_summary/"];
    NSData *jsonData = [NSData dataWithContentsOfURL:url];
    NSError *error = nil;
    NSDictionary *dataDictionary = [NSJSONSerialization JSONObjectWithData:jsonData options:0 error:&error];

    self.tajuk = [dataDictionary objectForKey:@"posts"];

//    NSLog(@"%@", dataDictionary);

    self.navigationItem.leftBarButtonItem = self.editButtonItem;

    UIBarButtonItem *addButton = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemAdd target:self action:@selector(insertNewObject:)];
    self.navigationItem.rightBarButtonItem = addButton;
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

- (void)insertNewObject:(id)sender
{
    if (!_objects) {
        _objects = [[NSMutableArray alloc] init];
    }
    [_objects insertObject:[NSDate date] atIndex:0];
    NSIndexPath *indexPath = [NSIndexPath indexPathForRow:0 inSection:0];
    [self.tableView insertRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationAutomatic];
}

#pragma mark - Table View

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    return 1;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    return self.tajuk.count;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"Cell" forIndexPath:indexPath];

    NSDictionary *dataFromWeb = [self.tajuk objectAtIndex:indexPath.row];

   // self.URL = [NSURL URLWithString:[dataFromWeb objectForKey:@"thumbnail"]];

//    NSString *object = self.tajuk[indexPath.row];
    cell.textLabel.text = [dataFromWeb objectForKey:@"title"];

   // NSLog(@"%@", [dataFromWeb objectForKey:@"thumbnail"]);

    if([[dataFromWeb objectForKey:@"thumbnail"] isKindOfClass:[NSString class]]) {

        NSData *imageData = [NSData dataWithContentsOfURL:[dataFromWeb objectForKey:@"thumbnail"]];
        UIImage *image = [UIImage imageWithData:imageData];
        cell.imageView.image = image;
        NSLog(@"%@", [dataFromWeb objectForKey:@"thumbnail"]);

    } else {

        cell.imageView.image = nil;

   }

    return cell;
}



- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    if ([[segue identifier] isEqualToString:@"showDetail"]) {
        NSIndexPath *indexPath = [self.tableView indexPathForSelectedRow];
        NSDictionary *dataWeb = [self.tajuk objectAtIndex:indexPath.row ];
        NSString *objek = [dataWeb objectForKey:@"author"];
        //NSString *object = self.tajuk[indexPath.row];
        [segue.destinationViewController setDetailItem:objek];
    }
}

@end

NSData *imageData = [NSData dataWithContentsOfURL:[dataFromWeb objectForKey:@"thumbnail"]];