How to change the top picture of blueprint theme for Drupal 6?

I am so new to this… I just need to change the top image of the website (link below) to another image. The website is using Drupal 6 and the blueprint theme. I am able to get to the theme building page, is that the right place to start?

Thank you so much! I will really appreciate all the help that I get!!

Drupal version: