Open the editor of the post or page where you want to display a unique custom header image, and locate the Featured Image module in the bottom right corner under Page Attributes. Click Set featured image.
If you don’t see the Featured Image module on your Add New or Edit Post page, please make sure you’ve selected Featured Image in your Screen Options.
You will see an uploading screen identical to that used when inserting an image into a blog post. Follow the on-screen prompts to choose the image from your computer and upload it or choose it from one of the images already in your Media Library.
Once the image has successfully been uploaded, look underneath the sizing options, and click Use as featured image: