I've done tests and found that if an image is not specified as the "Featured image" when writing the post via the dashboard, then the slider shows a blank white space. This happens even if there is an image attached within the body of the post. This doesn't help for those who may email in their post, hence skipping the dashboard and chance to put in the featured image.
My goal was to be able to have the slider show the either:
1. The featured image chosen via the dashboard when writing the post
2. If #1 not done, then use an image included within the body
3. If #2 none done, then use a specified default "catch-all" as the featured image.
However, it seems that when using the slider with full-size image (i.e., the featured image slider), then the author must choose the featured image or else all you get is a blank white space. :(