Hello
I am quite new to WP and may be my solution isn’t the best, but seems to work for me. It requires some basic knowledge of HTML/CSS.
Also, I am integrating WP to my site, no full site installation, but that does not change much the procedure, I believe.
Before the regular post contents and media, I added the teaser images to the post in post editing.
Using the text view, I added a html <div> tag around the image and gave it a class name.
I set the [more] tag where I need it.
Then, on the loop placed on the page where I want to show the teaser, short version, excerpt, whatever you call it, I set it to content, not excerpt.
Then, with CSS display property in your regular style sheet you can control whether or not that image shows, by setting it to display:none or allowing it its default, ie, it shows.
That’s it. In my case I have 2 different places with different sizes where I want to show the teasers, so I actually added 2 images to the post, that are shown each at the desired place.
It is not set up with WP, yet, but the result will be as you see at the homepage/blog page and posts at https://www.soeezauto.ma.
Hope that helps.