I found that the plugin created files in /public_html/wp-content/uploads. If you check your dashboard’s media library and “edit” the photo, it shows the location of the files there. If you’re like me, and the only thumbnails created are the ones by this plugin, you’d delete the year-dated folders in the /uploads section and then delete the Media Library’s thumbnails.
I know this because I was getting rid of them for a different reason and that’s how I went about it and they are completely gone.