bbPress Topic Thumbnails is a WordPress plugin by TheWebAtom that adds support for Topic Thumbnails with bbPress.
It works by retrieving the first image from each topic and displaying it alongside the topic title on the index page. Thumbnails are sized according to your media settings, however they are capped to a maximum height & width of 128px. This is to prevent the thumbnails from breaking bbPress ‘ topic display CSS.
This plugin works with GD bbPress Attachments and most other bbPress image management plugins.
Download: http://wordpress.org/extend/plugins/bbpress-topic-thumbnails/