BuddyPress.org

Assuming you mean the custom header that you find under appearance > header then here is a blog post with a tutorial.

http://buddydress.com/2010/04/custom-header-tutorial-adding-an-image-and-changing-the-size/

Excuse just linking but it pretty much sums up easier than me saying to you what to do and hopefully will help as a reference.

If it’s not custom header just let me know.