Just edit the link in header.php in your template folder to point it to your WordPress home page, rather than the bbPress home page.
So instead of this:
<a href="<?php bb_uri(); ?>"><?php bb_option('name'); ?></a>
You would have something like this:
<a href="http://www.example.com/home/">WordPress Home</a>
I don’t want text in there though, I want to make the entire header image a link. The image is defined in the CSS.
If it’s a background image defined in the stylesheet, you need to do something like this:
http://www.wordpressmax.com/wordpress/wordpress-header-link
https://wordpress.org/support/topic/181216
http://www.penny4them.com/2008/clickable-header/
The instructions are for WordPress, but you’ll need to use a similar procedure. If you don’t want to do it with JavaScript, you need to move the image into the template file and call it with an <img tag, not define it as a background image for the div in the stylesheet.