Facebook graph api Uitableview照片显示，然后变为黑色

我目前正在使用一个UITableview，它从Facebook加载一个用户墙提要。我遇到的问题是，放入tableview的图像加载得很好，但当用户滚动时，它们会变成完全黑色。以下是我的cellForRow方法：

 if ([[objectTypes objectAtIndex:indexPath.row] isEqualToString:@"photo"]) {
    //this is a picture

    if ([facebookPictureObjectIDs objectAtIndex:indexPath.row]) 
                    {
                        if ([sourceArray objectAtIndex:indexPath.row]) {
                            if ([[sourceArray objectAtIndex:indexPath.row] rangeOfString:[facebookPictureObjectIDs objectAtIndex:indexPath.row]].location != NSNotFound) {

                                imageCell.imageview.image = [UIImage imageWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:[sourceArray objectAtIndex:indexPath.row]]]];
                                NSLog(@"source array objectwith image ID: %@",[facebookPictureObjectIDs objectAtIndex:indexPath.row]);


                            }else {


                                NSLog(@"Same Image object id is: %@", [facebookPictureObjectIDs objectAtIndex:indexPath.row]);


                            }

                        }   else {
                            NSLog(@"Source Array object NIL NIL NIL");

                        }                 

                    }else {
                        NSLog(@"Else Called");
                    }

    [imageCell layoutSubviews];
    returnCell = imageCell;
   }

谢谢你的帮助：）