Tested on a premium theme on wp-3.81 WORKS!

Heads up for anyone needing this on 3.81. I tested it locally and now using n a production site. It does work, just make sure you upload images with names like EW-23.jpg or EW-myimage.jpg It makes all this clear in the readme, so should not be a problem. The upload is done using WP native ‘add media button’ Does not need to be post/ page specific, as it searches for the EW- prefix. The plugin is simple and lightweight, one short file in fact. If you decide to add your own featured image, the plugin steps out of the way and allows that image to override. Otherwise, it adds featured images that you have uploaded with the EW- name prefix at random. If those featured images are used elsewhere, ie in widget thumbs, they all get resized as you would expect. All seems good to me. Pity the developer does not update the version compatibility and give this one a new lease of life.