Aw, you know what, I went about that for a similar plug-in called generate-post-thumbnails. I tried Auto Post Thumbnail but it didn’t seem to work for me. Sorry about the mix-up, the two are so similar and I installed them at the same time.
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.
