I don’t understand what you just said.
The post-header insertion area comes right after the header area, and will follow the #wrapper margins/padding. It comes before anything in the content area.
The container-top area is then next content emitted after the post-header insertion area, except it is inside the container wrapping div. There is nothing else in between.
</div><!– #header –>
<div id=”inject_postheader”>
… postheader content
</div><!– #inject_postheader –>
<div id=”container” class=”container container-page relative “>
<div id=”inject_container_top”>
.. container-top content
</div><!– #inject_container_top –>
It doesn’t matter what is the content of either of those areas – image/slider, HTML, shortcode, whatever. There is no intervening output.