JSONブロブからロードしている画像でも同じ問題がありました。GCD を使用して、各セルに割り当てられたキーとペアになった NSDictionary に画像を保存しました。
- (UIImage *)imageForRowAtIndexPath:(NSIndexPath *)indexPath
{    
    // get the dictionary for the indexPath
    NSDictionary *tweet = [tweets objectAtIndex:[indexPath row]];
    // get the user dictionary for the indexPath
    NSDictionary *user = [tweet objectForKey:@"posts"];
    //get the image URL
    NSURL *url = [NSURL URLWithString:[[[[[tweet objectForKey:@"photos"] objectAtIndex:0] objectForKey:@"alt_sizes"] objectAtIndex:3] objectForKey:@"url"]];
    // get the user's id and check for a cached image first
    NSString *userID = [user objectForKey:@"id"];
    UIImage *image = [self.images objectForKey:userID];
    if(!image)
    {
        // if we didn't find an image, create a placeholder image and
        // put it in the "cache". Start the download of the actual image
        image = [UIImage imageNamed:@"Placeholder.png"];
        [self.images setValue:image forKey:userID];
        //get the string version of the URL for the image
        //NSString *url = [user objectForKey:@"profile_image_url"];
        // create the queue if it doesn't exist
        if (!queue) {
            queue = dispatch_queue_create("image_queue", NULL);
        }
        //dispatch_async to get the image data
        dispatch_async(queue, ^{
            NSData *data = [NSData dataWithContentsOfURL:url];
            UIImage *anImage = [UIImage imageWithData:data];
            [self.images setValue:anImage forKey:userID];
            UITableViewCell *cell = [self.tableView cellForRowAtIndexPath:indexPath];
            //dispatch_async on the main queue to update the UI
            dispatch_async(dispatch_get_main_queue(), ^{
                cell.imageView.image = anImage;
            });
        });
    }
    // return the image, it could be the placeholder, or an image from the cache
    return image;
}