bbPress.org

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.