WordPress Automatic plugin can auto post from almost any website to WordPress posts and it can also set the featured image from PixaBay for a specific keyword, here are the steps to get your PixaBay API key

1- Login to your PixaBay account here, sign up if you do not have an account

https://pixabay.com/

2- Visit this link https://pixabay.com/api/docs/ and scroll till you find your API Key