Adding a Featured Image to your blog post
A common thing I find with websites that have been built by people who are picking it up as they go along, is that they dont know about adding featured images.
Featured images can be added to blog posts, and pages, amoungst other things, but it is blog posts where they are best used. WordPress uses these featured images to display in the lists of blog posts , known as archives, and it is often this blog post list/archive that poeple first see. Having that featured image displayed is a huge part of capturing someones attention, and getting them to click on that link to read the post.
How to add a featured Image
The mistake that people make , is to include their image into the body of the content, in the way I have added the image at the top of this post. They add this to the top of their content, and it will be displayed in the blog post, but not on that list.
To add a featured image, from your wordpress dashboard, edit your page. On the right hand side you should see the option to set the featured image. Check that you are in the post tab, not the block tab. You may need to scroll down a bit. Click on the grey box where its says set featured image, and select one from your media or upload in the ususla way. Dont forget to update or publish the post.

I have created a Youtube video showing you how to do this, step by step.