Featured images (also called Post Thumbnails or Image Post) are images that you can attach to your blog posts that represent your content. When posts are shared on social media or displayed as a grid on your blog, they usually have the title of the post as well as a picture. This picture is the Featured Image and is configurable through the Edit Post menu.
Each theme has different specifications (size, location, etc) for how it will display the featured images. Some themes will only show the featured image on a page that has a grid or list of your posts and others will also show the featured image above the content of your posts.
If your theme displays your blog posts in a grid (like our example at the top), the image you attach to your posts will need to be large enough that it doesn’t look grainy or stretched. Also, if you want it to be sharable on social media it will need to be at least 900px wide.
As a rule of thumb, take a look at the documentation for your theme to ensure you choose the right size before you upload your image. Trial and error is also a great way to find the right dimensions for your specific theme. Everything can be changed and updated so don't worry too much about making the wrong choice to start.
In the Edit Post screen of your blog post there is a section in the right-hand sidebar that you can use to attach a featured image to your post. It uses the same media library system that you will use when inserting photos into your content.