Laura530,
I just added this post to my site: https://shapeshifter3.com/music-videos/sugar-by-maroon-5/ which includes a New Featured Image. I’ve changed NO other images or re-uploaded any throughout the site. So far, I’ve come across no problems whatsoever. If you Left-click on Music Videos in the Primary Menu; drag the right edge of your screen towards the left on either a Desktop or Laptop; all Featured Images seem to display correctly.
The purpose of the Picturefill components of this plugin is to correctly distribute the proper image-size to whatever mobile device an end-user owns. After going through my Admin Panel I’ve found no visible changes from the plugin. I assume multiple image resizing takes place on the hosted server. I’m just not sure in which file they are located.