对系统图片进行放大或者缩小

   UIImage *image = [UIImage imageNamed:@"用户头像120px.png"];

        CGSize size = CGSizeMake(15, 15);

        UIGraphicsBeginImageContextWithOptions(size, NO, 0.0);

        CGRect rect = CGRectMake(0.0, 0.0, size.width, size.height);

        [image drawInRect:rect];

        cell.imageView.image = UIGraphicsGetImageFromCurrentImageContext();

        UIGraphicsEndImageContext();