Customise Blog Header / Insert Image block

I have created a blog post, but it wont let me add an image block to the header (like the rest of the site) so the Logo and Nav (which are white) have disappeared on the white background! The Newsroom/Blog templates are as if they are 'locked' and I can't add any Image/code blocks to those pages?

Also on the Newsroom overview page I cant add any code blocks to add an anchor for the article links. 

Any help appreciated please! 

Thank you

We can use CSS code to add image or change header color. Can you share link to a blog post? We can check easier