mbecher

If your theme supports the thumbnail feature introduced in WordPress 2.9, this plugin will display attached thumbnails on the admin area’s manage posts and manage pages screen, similar to the media management page.

Admin Thumbnails

If your theme supports Wordpress 2.9's thumbnail feature, this plugin displays thumbnails on the admin area's manage posts and manage pages  …

Shane Gowland

This plugin 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 the default ‘thumbnail’ size in your WordPress media settings. The images will also resize appropriately if you’re using a responsive WordPress theme.
This plugin works with GD bbPress Attachments and most other bbPress image management plugins.
If you enable this plugin while bbPress is disabled (or not installed) it will display a warning message.

bbPress Topic Thumbnails

Provides bbPress with support for topic thumbnails.

Admin Thumbnails vs bbPress Topic Thumbnails