Hi Livsy!
That’s weird. All those thumbnails are not generated by our plugin, but by your theme. Any posts using an external featured image will use one of those thumbnails as a “frame” in which the actual external image URL will be set.
If you’re having trouble with the 1.3 series, I recommend you go back to version 1.2.0. In fact, I’ve set version 1.2.0 as the stable one, so that nobody else faces any more problems.
I’ll keep working on the 1.3 series and, hopefully, I’ll be able to fix all those issues. In the meantime, though, it’ll be better if you stick to the stable version.
Best,