如何下载照片？

我有一个包含4张照片，我从网络中这些照片，但问题是，当我向下滚动，这些照片被再次下载 enter image description here 如何下载照片？

中的UITableView这是代码一个UITableViewCell：

 ITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"cellPhoto"]; 

if (cell == nil) { 

    NSArray *nibObject = [[NSBundle mainBundle] loadNibNamed:@"CustomCellThumbnails" owner:self options:nil]; 
    cell = [nibObject objectAtIndex:0]; 
} 

// Get the photos 
int getPhotos = indexPath.row * 4; 

for (int i = 1; i <= 4; i++) { 

    if (getPhotos < [imageArray count]) 
    { 
     UIButton *imageButton = (UIButton*)[cell viewWithTag:i]; 
     NSString *url = [imageArray objectAtIndex:getPhotos]; 

     dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ 
      NSData *imageData = [NSData dataWithContentsOfURL:[NSURL URLWithString:[NSString stringWithFormat:@"http://www.siteweb.com%@",url]]]; 
      UIImage *imageFieldProfile = [[UIImage alloc] initWithData:imageData]; 
      dispatch_async(dispatch_get_main_queue(), ^{ 
       // Set the phoyo to the UIButton 
       [imageButton setBackgroundImage:imageFieldProfile forState:UIControlStateNormal]; 
       [imageFieldProfile release]; 
       // Set the corner of UIButton 
       [imageButton.layer setCornerRadius:5.0]; 
       [imageButton.layer setMasksToBounds:YES]; 
       imageButton.tag = getPhotos; 
      }); 
     }); 

     [imageButton addTarget:self action:@selector(displayPhoto:) forControlEvents:UIControlEventTouchUpInside]; 

    }   
    getPhotos ++; 
}

来源

2011-12-31 keep on you