Header background image is fluid/responsive, it is meant for generic image (abstract, scenery etc.). DO NOT upload a well-designed image for this area, as it will respond to screen resolution and do not show as a fixed image size.

1- Go to Appearance – Theme Options
2- On General settings tab find Header background image
3- Click Upload to select a file and upload

Recommend size of the image is 1600px (width) x 420px (height) or larger

4- Click Save settings