Cells are hidden using CollectionView

Question

I have a Navigation Controller pushing to a Collection View Controller. This collectionView has an NSArray of images, and I'm sure there are some elements in the collectionView using the method visibleCells. But when I launch it there is no displaying cells. Here is my code :

- (void)viewDidLoad
{
    [super viewDidLoad];
images = [NSArray arrayWithObjects:@"angry_birds_cake.jpg", @"creme_brelee.jpg", @"egg_benedict.jpg", @"full_breakfast.jpg", @"green_tea.jpg", @"ham_and_cheese_panini.jpg", @"ham_and_egg_sandwich.jpg", @"hamburger.jpg", @"instant_noodle_with_egg.jpg", @"japanese_noodle_with_pork.jpg", @"mushroom_risotto.jpg", @"noodle_with_bbq_pork.jpg", @"starbucks_coffee.jpg", @"thai_shrimp_cake.jpg", @"vegetable_curry.jpg", @"white_chocolate_donut.jpg", nil];
} 

- (NSInteger)collectionView:(UICollectionView *)collectionView numberOfItemsInSection:(NSInteger)section {
return images.count;
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{
static NSString *identifier = @"Cell";

UICollectionViewCell *cell;
[collectionView registerClass:[UICollectionViewCell class] forCellWithReuseIdentifier:@"Cell"];
cell = [collectionView dequeueReusableCellWithReuseIdentifier:identifier forIndexPath:indexPath];


NSArray *tab = [collectionView visibleCells];
for (int i = 0; i < [tab count]; ++i)
{
    NSLog(@"%d", i);
}
[collectionView setBackgroundColor:[UIColor blueColor]];

UIImageView *imageView = (UIImageView *)[cell viewWithTag:100];
imageView.image = [UIImage imageNamed:[images objectAtIndex:indexPath.row]];


return cell;
}

Where am I wrong ? Thanks a lot.