From Georgetown University Commons Knowledge Base
To change the header background image, remove the header image, or delete the Blog Name and Description from the header, select "Header" under the "Appearances" menu.
This will open a menu on the left side of the screen. From here, you can preview any changes to your blog before you save them. To upload a new photo click on "Add new image." The file must already be saved on your computer or in your Media Library. After you click upload, it will then ask you to crop the image you have uploaded.