Actually you do not have do anything wrongly, just the truncation part it will not show the image on the blog listing page by default, unless you have put the post cover image, so it will show the post cover on the blog listing page.
What I trying to explain is whether you have put the post cover or not, it doesn't matter, it really depend on the user how he put the content from their blog post.
For now, I already help you configured the correct setting, it will show 1 image on your blog listing page.