We posted an article about how to set a default fallback image for WordPress post thumbnails in which we explained how you can add some code to your WordPress theme in order to have default fallback post thumbnails. One of our readers pointed us to a plugin that does the same thing except that it also gives you a backend visual display of the image you selected. This is a solution for beginners who do not want to get their hands dirty with the codes.
First, install and activate the Default Post Thumbnail Plugin. Go to Settings » Default Post Thumbnail. You should see a screen asking you to select which thumbnail you want to set as default.
All you have to do is pick one, save your settings, and you are good to go.