Hmm, I’m not sure what could be causing that.
We’re deprecating the current image size features in 1.2.0: https://wpshowposts.com/wp-show-posts-1-2-0/
Instead, you should create an image size using this method: https://docs.generatepress.com/article/adding-image-sizes/
Then if you use 1.2.0-alpha.1, you can choose the image size (if you remove the fixed values and save).
This is better for performance and uses core WP functionality 🙂