Featured images

Squazz,

While you are waiting on a response, you might read these two posts:
https://css-tricks.com/responsive-images-youre-just-changing-resolutions-use-srcset
Official Responsive Images Plugin for WordPress

AND, here’s a GitHub link to version 2 of what you’re currently using:
https://github.com/kylereicks/picturefill.js.wp/tree/master

AND, a GitHub link to the recommended RICG Responsive Images Plugin:
https://github.com/ResponsiveImagesCG/wp-tevko-responsive-images

Personally, I would like to see the developer of your current plugin join the ranks as a contributor to the RICG Responsive Images Plugin and add his own expertise. That way he could contribute only when he has the time, and not be obligated to your plugin. Why not pool creative resources, and provide the best product available for the end user (us).

The RICG Responsive Images Plugin works for my featured images, but not for attached images in my posts. And, I’m having problems seeing how I can add lazy load to images handled by the RIGG plugin. (But this will be mentioned in the thread for the other plugin)