If you show thumbnails on your wordpress blog home page, you need to upload these thumbnails for each post when you are posting an article.
You can also set to use any image of the article as the thumbnail. But what if you don't have any images in your article and you don't have any thumbnail or you forgot to upload thumbnail for your post. In those cases, this video is helpful for you.
You can set some default images for the categories you have in your blog and whenever you post any article under those category and if you don't have a thumbnail for the post then the category image will be used as the thumbnail.
The plugin used for this is "FPW Category Thumbnails". Using this plugin you can upload default images for your categories and these will be used as the default thumbnails for your posts.
In this video I have shown how to use this plugin.
On this page of the site you can watch the video online Default Post Thumbnails in Wordpress| FPW Category Thumbnails with a duration of hours minute second in good quality, which was uploaded by the user Blogging My Passion 28 March 2016, share the link with friends and acquaintances, this video has already been watched 13,353 times on youtube and it was liked by 71 viewers. Enjoy your viewing!