For each post, you can define an image that will be displayed along with your content on Posts and Category pages. Thumbnail images (aka Featured Images) are added using the “Featured Image” option within your WordPress Dashboard. Whenever you create or edit a post, look for the “Featured Image” box.
This box will be located to the right of your text editor or below your text editor depending on the size of your screen. Click the “Set featured image” link and upload the image that you would like to use. Once uploaded, scroll down and select “Full Size” as your image size, and then click the “Use as featured image” link. This image will now be used as your Thumbnail image for the post.
Important Note: The WordPress thumbnail function re-sizes the image you upload based on what thumbnail sizes are needed for your theme. Each time you change a theme, you may need to re-generate these thumbnails so that they fit your new theme correctly. You can use the Regenerate Thumbnails plugin each time you switch themes to re-generate your thumbnails. After enabling this theme, you should run this plugin to update all of your old thumbnails to avoid design inconsistencies.