To add a custom header image to your blog, display just text or display text and and an image, head over to the Appearance > Header page in your blog's dashboard. The video below will walk you through the basics of adding a header. For more information, check out the Custom Header Image page on WordPress.com.
Note: The first 30 seconds of the video walk you through selecting a theme that supports custom headers. Since all Argo themes support custom headers, you can ignore this portion of the video.