UICollectionViewで非同期に画像を取得して表示 ref: http://qiita.com/ocadaruma/items/6bec7366b1a3c63d7467

CustomCell.h

#import <UIKit/UIKit.h>

@interface CustomCell : UICollectionViewCell

@property (weak, nonatomic) IBOutlet UIActivityIndicatorView *indicatorView;
@property (weak, nonatomic) IBOutlet UIImageView *imageView;

@end

ViewControler.m

#import "ViewController.h"
#import "CustomCell.h"

static NSString* const kCellIdentifier = @"cell";

@interface ViewController ()<UICollectionViewDataSource, UICollectionViewDelegate>

@property (weak, nonatomic) IBOutlet UICollectionView *collectionView;
@property (nonatomic) NSCache* imageCache;
@property (nonatomic) NSOperationQueue* queue;

@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];

    [self.collectionView registerNib:[UINib nibWithNibName:@"CustomCell" bundle:nil] forCellWithReuseIdentifier:kCellIdentifier];
    self.imageCache = [[NSCache alloc] init];
    self.queue = [[NSOperationQueue alloc] init];
}

- (UICollectionViewCell *)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath
{
    CustomCell* cell = [collectionView dequeueReusableCellWithReuseIdentifier:kCellIdentifier forIndexPath:indexPath];

    cell.imageView.image = nil;
    [cell.indicatorView stopAnimating];

    UIImage* image = [self.imageCache objectForKey:indexPath];
    if (image) {
        cell.imageView.image = image;
    } else {
        [cell.indicatorView startAnimating];

        NSURLRequest* request = [NSURLRequest requestWithURL:[NSURL URLWithString:@"画像のURL"]];

        [NSURLConnection sendAsynchronousRequest:request queue:self.queue completionHandler:^(NSURLResponse *response, NSData *data, NSError *connectionError) {
            CustomCell* aCell = (CustomCell *)[collectionView cellForItemAtIndexPath:indexPath];
            UIImage *aImage = [UIImage imageWithData:data];

            [aCell.indicatorView stopAnimating];
            [self.imageCache setObject:aImage forKey:indexPath];
            [collectionView reloadItemsAtIndexPaths:@[indexPath]];
        }];
    }

    return cell;
}

@end