从JSON中的URL加载的图像视图在滚动时会发生变化

我正在将JSON文件中的图像URL异步加载到UITableView上的图像视图中。图像加载适当，但当我向下滚动订阅源时，用户的个人资料图片会更改为默认头像（当用户尚未加载个人资料照片时使用）。我的php有一个必要的if语句，用于首先检查是否有与该用户相关的配置文件图片。当代码看到没有关联的照片时，它自然会加载默认的化身。

所以。。。当应用程序启动时，我会看到提要和（几秒钟后……也许也有帮助？）提要加载中的图像，但当我滚动经过它们并返回时，它们现在显示为默认化身，并慢慢变回相应的关联图像。我真的不知道从哪里开始。。。这是缓存问题吗？不应该在加载页面之前加载图像吗？非常感谢您对修复我的代码的任何帮助！

我的PHP文件：

$posted="posted";
$query1 = $conn->prepare('SELECT * FROM users WHERE ID=:id');
$query1->bindParam(':id', $_SESSION['uid']);
$query1->execute();
$row = $query1->fetch();
$username="dvdowns";
$zero=0;
$posted="posted";
$query = $conn->prepare('
SELECT description, city, status, state, christmas, country, 
needsusername, howmanypeopleneeded, howmanypeoplesignedup, 
needs.orgname, needs.ID, titleofneed, expiredate, datesubmitted 
FROM needs INNER JOIN follow ON follow.followname = needs.needsusername 
WHERE follow.username = :username AND needs.christmas=:zero 
AND needs.status=:posted ORDER BY datetime DESC LIMIT 0, 10');
$query->bindParam(':username', $username);
$query->bindParam(':zero', $zero);
$query->bindParam(':posted', $posted);
$query->execute();
$arr = array();
while ($rows = $query->fetch()) {
    $title=$rows['titleofneed'];
    $description=$rows['description'];
    $needsusername=$rows['needsusername'];
    $query1 = $conn->prepare('SELECT * FROM users WHERE username=:username');
    $query1->bindParam(':username', $needsusername);
    $query1->execute();
    $row = $query1->fetch();
    $photo=$row['photo'];
    if ($photo=="") {
        $photo = "http://domain.com/images/default.png";
    }
    else {
        $photo="http://www.domain.com/".$photo;
    }
    $arr['needTitle'] = $title;
    $arr['needPoster'] = $needsusername;
    $arr['needDescrip'] = $description;
    $arr['userImage'] = $photo;
    $data[] = $arr;
}
echo json_encode($data);

我的TableViewController.m文件：

    NSURL *myURL = [[NSURL alloc]initWithString:@"http://domain.com/json2.php"];
    NSData *myData = [[NSData alloc]initWithContentsOfURL:myURL];
    NSError *error;
    jsonArray = [NSJSONSerialization JSONObjectWithData:myData options:NSJSONReadingMutableContainers error:&error];
    [tableView reloadData]; // if tableView is unidentified make the tableView IBOutlet
}
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    // Return the number of rows in the section.
    return jsonArray.count;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    NeedCardTableViewCell *cell = (NeedCardTableViewCell *) [tableView dequeueReusableCellWithIdentifier:@"needCard" forIndexPath:indexPath];
    NSDictionary *needs = jsonArray[indexPath.row]; // get the data dict for the row
    cell.textNeedTitle.text = [needs objectForKey: @"needTitle"];
    cell.textNeedPoster.text = [needs objectForKey: @"needPoster"];
    cell.textNeedDescrip.text = [needs objectForKey: @"needDescrip"];
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
        NSURL *imageURL = [NSURL URLWithString:needs[@"userImage"]];
        UIImage *image = [UIImage imageWithData:[NSData dataWithContentsOfURL:imageURL]];
        dispatch_async(dispatch_get_main_queue(), ^{
            [cell.imageProfPic setImage:image];
        });
    });
    return cell;