You can add a header image to make you blog more attractive.
NB different themes may work differently. For example they need different sized images.
Accessing the Header Editor
On your blog’s dashboard, got to Appearance-> Header
or on the hover over the blog name  in the Admin bar and choose Header.
Header Section of The Customiser
The step above opens the Header Section of the Customiser.
Note that you are told the size of image that is needed .
Click Add new image. .
Choose an Image
The Media Library Will open, you can choose an exiting image if any or Select Files from your computer.
Pick a file from your computer 
The image will upload and selected. Choose Select and Crop .
Next you will crop the image. The selection will be at the correct aspect ratio, it is best just to move this and then click Crop Image.
Preview and Save
The Crop image dialog will close and you will be able to preview your header. If you are happy Click Save & Publish and then close the customiser.