Thank you for the answer!
to elaborate my questions:
1, thanks for teaching my how to add the header image and I tried it well, but it seems that this image only shows on the blog-post page. What if I want to add to every different page a different header image?
2, by the question I mean, I follow the documentation (https://mahdiyazdani.github.io/Hypermarket/#/setup-homepage-template) and this suggestion:
"Remember, if you want to have a page to display your blog posts too, you will have to create another page (you can leave the content blank) and set that as the Posts Page in the same settings section like above where we created a blank page titled Blog on our site and set that as the posts page."
Then on this posts page, in my site that is https://yu-xuan-li.com/solilogue-post/, it shows all posts that I have created. However, what I want is to show only some of the posts, for example a bundle of them with tag "poem", then how can I do this?