If you find you do not have the correct level of access to carry out this task, please contact support for this WordPress.
To change the header image for a single page or post only:
STEP 1: prepare your image. You will need an image file of at least 1000 pixels width, but it must also be under 1Mb (the maximum upload size). If you don’t know how to do make an image file like this, perhaps ask a photoshop guru.
STEP 2: edit the page or post for which you want to set this as the header image
STEP 3: look for the ‘Featured Image’ panel near bottom right of the editing page. Click ‘Set Featured Image’
STEP 4: drag and drop your image file to upload it
STEP 5: after the image uploads, towards the bottom of the image details you should find a link called ‘Use as featured image’ – click this, wait while this task is carried out, then DO NOT insert the image on your page or post, instead simply click the X top right of the window you’ve just used to close it.
STEP 6: click Update (or Publish) to save your page or post.
STEP 7: click to view or preview your page or post to see the result
note: the image size used for header pictures most sites here is 1000 x 187 pixels. If you want to control precisely what part of a larger image is shown, try cropping your image to this size before uploading.
To remove a ‘featured image’
Edit the page or post where the image appears. Find the ‘Featured Image’ panel towards the bottom right, and click ‘Remove featured image’. Alternatively, you can open Media>Library and delete the featured image file.