I noticed this problem when a user kept posting giant images. I looked inside all of these image names and noticed they all had "&" , an ampersand inside the image names. SO what is happening is a thumbnail is being created, i checked on my server, however when it goes time to execute the shortcode the file listed below executes no thumbnail.
I found this file buddypress-activity-plus/lib/forms/images_tag_template.php which looks like where it is grabbing the images. it looks like this line returns false, which doesnt make much sense.
file_exists(bpfb_get_image_dir($activity_blog_id) . $info['filename'] . '-bpfbt.' . strtolower($info['extension']))
and then it echos the incorrect filename.