How to add a logo or an image to the header?

To resolve this issue, below is our instruction:
Firstly, follow the jetpack’s guideline in the Blog article on how to use Edit CSS here.
Then add the code to the Edit Css.

.banner .page-title {
 text-indent: -9999px;
 background: url('FILE URL');
 width: 250px;
 height: 250px;
 border: none !important;

.style-flat .banner hgroup:after{
 display: none;

Note: You can add your logo image to the Media then copy the URL FILE on the right hand side.

This answer is used for the desktop/laptop screens, not for mobile, tablet screens.