How to Add a Header Image in Multi WordPress Theme?
Add custom background images to your header
In order to do that, from your WordPress Dashboard go to Appearance > Customize > Header Image and choose an image from your WordPress Media Library or upload a file directly from your computer.
You’ll be able to crop your image once you upload it for a perfect fit.
You can upload many images from your Media Library to Header Images and even randomize their display. This way each time the page is refreshed, a different image will appear as your header’s background! In order to do that, select the Randomize uploaded headers option.