I just need to change the top image of the website 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?
First of all: You shuldn't use Drupal 6 anymore as it has reached EOL (https://www.drupal.org/drupal-6-eol).
The regular way to replace the page logo is through the admin interface. You should have an logo file field on your theme settings page.
Another way is to check out the logo path with with firebug (or F12 for the regular browser console) and replace it directly with your ftp client.
Edit: If "Top Image" means something else then the logo, you can also use the second method, but make sure the determined image is not processed through imagecache, otherwise it will be overridden after the next cache clear.