How to add custom image to blog post excerpts?

I have been trying to set custom images for my blog post excerpts. So far, the excerpts inherit the featured image that is set for the post. I want to be able to set custom images for excerpts.

Can somebody help assist on how i can achieve that?

*For example, you can check this blog backlinko to see what am talking about. Most of the images attached to the excerpts are custom

Am on GP Premium 1.7.2

Thanks for any helpful replies.