Problem adding an image to the header in F8 Lite theme

Hi, There are two things I want to achieve in my website. I want to replace the blog title to an image (a banner actually); I need to increase the height of the header so as to fit in the image. Please see https://www.sudongyue.com.

I know there are many threads talking about this, but somehow I cannot get it done. I think this issue is theme specific. I am using F8 Lite. What I have done is that, first of all, I upload the image to the media in my wordpress dashboard. Then I add the image code in the header.php right under the id=”masthead”. It does not work. In another attempt, I change the CSS by simply adding #masthead { url of the image }. It does not work neither.

Please help!