If you would like to change the image on YOUR OWN post or page without changing the entire blog, follow these steps.
1. Go to the “Appearance” menu item in the Dashboard
2. Find the “Header” option in the menu list
3. Note the exact pixel size of the Header. DO NOT CHANGE THE HEADER.
After you write your page or post, find an image that either matches or exceeds the header size for the blog theme.
Use the “Add Media” button to open the Media Library:
Upload your correctly sized image using the upload tool on the Library page.
In the left-hand menu, select “Featured Image.”
Select the image you just uploaded from the Library. Publish your page.
Although the image does not appear in the page or post, you will see it shown as the featured image in a toolbox next to your editing window. It will replace the header image on your page when you view the post.