Hi there,
I’ve checked the WPSP on your page and I actually see an image on the automatically generated excerpt – https://share.getcloudapp.com/12u4nmrj
I’m not sure I fully understand. Are you pertaining to WordPress’ manual excerpt?
As for adding image through CSS:
It’s definitely possible.
Here’s an example of adding image through a pseudo element –
.wp-show-posts-entry-summary:after {
content: url(https://wordpress-554138-2025384.cloudwaysapps.com/wp-content/uploads/2021/07/socios-fundadores-branded-content-1-1.png);
}