这样也可以，insert,,

viewwillappear 之后访问  以autosizeing为 基准的 view  的frame，  

UIImageView *mImageView = [[UIImageView alloc]init];

    [mImageView setImage:[[UIImageimageNamed:@"activities_input_bg.png"] stretchableImageWithLeftCapWidth:10topCapHeight:10]];

    mImageView.frame = CGRectMake(0, 0, self.mScrollView.frame.size.width, self.mScrollView.contentSize.height);

    [self.mScrollViewinsertSubview:mImageView belowSubview:picView];