This is not what I mean, this removes the multi coloured stripes at the top of the page. I want to remove the first big picture in the blog message (the feature image that is selected for the blog post).
I succeeded in doing this for the pages with the following ccs update:
.page img.attachment-full.size-full.wp-post-image {
display: none;
}

But this does not work for the blog posts.