BuddyPress.org

Oh, I see the ‘recent posts’ widget. Ok.

You’ll have to change line 36 in bp-blogs-widgets.php which looks like this now:

<a href="<?php echo bp_post_get_permalink( $post, $post->blog_id ) ?>" title="<?php echo apply_filters( 'the_title', $post->post_title ) ?>"><?php echo bp_core_get_avatar( $post->post_author, 1 ) ?></a>

That spot at the end there <?php echo bp_core_get_avatar( $post->post_author, 1 ) ?> is the author’s avatar. Replace that code with something that detects an image thumbnail if it exists and switch between the thumbnail and the author avatar.