In the below code whenever I am scrolling the tableview, images in each cell are changing, which shouldn't happen. Please help. Thanks in advance. 

```
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath{
UserDetails *userDetails = [arrUserDetails objectAtIndex:indexPath.row];
static NSString *CellIdentifier = @"CustomCell";
__weak TableViewCell *cell = (TableViewCell *)[_tableViewUsername dequeueReusableCellWithIdentifier:CellIdentifier];
cell.tag = indexPath.row;
cell.userName.text = userDetails.userName;
   [self.operationQueue addOperationWithBlock: ^ {  
    NSURL *aURL = [NSURL URLWithString:userDetails.userImageURL];
    NSError *error = nil;
    NSData *data = [NSData dataWithContentsOfURL:aURL options:nil error:&error];
    UIImage *image = nil;
    if (cell.tag == indexPath.row)
    {
        image = [UIImage imageWithData:data];
        [[NSOperationQueue mainQueue] addOperationWithBlock: ^ {
            cell.customImageView.image = image;
            cell.customImageView.contentMode = UIViewContentModeScaleToFill;
            cell.accessoryType = UITableViewCellAccessoryCheckmark;
        }];
    }
}];

```