You will be able to set a title for the blog post, add the body content, also provide all the meta information such as extra details (Categories and Tags) and media (Featured Image). In that meta blog you can set the image for the thumbnail.
Step by step :
1. Enter a title in the column that has been provided.
2. Fill the text editor with your content.
3. Add image here and the image will display as featured image.
4. Publish it!