If you know a little CSS and you’re not afraid the change the bbPress templates a little you can make this happen with through the forum_id(); which is unique for each forum.
Take a look inside loops-forums.php ( the template that starts the forums loop ) you’ll find this. <?php bbp_forum_id(); ?>
Copy that, next open your loop-single-forum.php and paste that code where you would like your image to show up.
For example something like this could be possible.
<img class="bbp-featured-img" src="http://example.org/images/forums/featured_forum_<?php bbp_forum_id(); ?>.png" alt="<?php bbp_forum_title(); ?>" /><a class="bbp-forum-title" href="<?php bbp_forum_permalink(); ?>"><?php bbp_forum_title(); ?></a>
All you have to do now is make sure you call your forum images featured_forum_[ID].png and upload them to the path you set in the image tag.
If you Hover over the forum names at the WP back-end you’ll see where the URL-page is pointing to if you would’ve clicked it, this link has a number like 88 or something, each forum has an unique number so your could pick that number and call it in your images, featured_forum_88.png >> upload the image and it will show up next to the forum title.
Change-/position it a little by CSS ( img.bbp-featured-img { } ) and you’re done.
I’ve done this myself the same way only with icon-fonts in front of the forum titles.
It’s a little bit of customization and it has a static feeling but it’s not that difficult and works perfect.
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Edit
Almost forgot to say, don’t change the bbPRess template files inside your plugins directory but copy those files to your theme directory.
https://codex.bbpress.org/theme-compatibility/