1. Open http://www.your-wordpress-blog-url.com/wp-admin
2. Click on Appearance
3. Click on Editor (in Appearance)
4. From the right file list: click on Header (header.php)
5. Go to end of code line: <p class="description"><?php bloginfo('description'); ?></p>
6. Click on Enter to open new empty line
7. Write and Click on Enter again
8. Write the banner HTML line:
<div align="center"><a href="your_affiliate_link/or other URL" target="_blank" rel="nofollow"
title=""><img src="your_image_url" width="" height="" alt="" title=""/></a></div>
9. You should see the end tag </section> after your banner link
10. Click on 'Update File' button
Note: If it don't work, you can play with the location of your banner code,
because it depend on your theme code.