The sermon image will show up if there is an image associated with the sermon. The image is chosen by priority:
1. featured image
2. series image
3. preacher image
If there isn't an image it shows as blank. How your theme displays featured images will obviously potentially cause an issue. You could hide one of the images with CSS or by creating your own custom single-wpfc_sermon.php file in your theme. Just copy single.php and rename it.