You can go to Appearance » Header to add your header image.
It will take you to the Customizer window. You can click on the Add New Image button to add the header image.
Then, a window will pop up that shows your Media Library images.
You can choose from a previously uploaded image, or drag and drop to upload an image from your computer. Then hit the Select and Crop button.
WordPress allows you to crop and resize your large header images as needed for your website.
Note that header image only works on blog post page template.