Depending on which theme you’re using, post thumbnails are set to show up in the index pages, or most of the time – in the single post pages. Adding a post thumbnail to your post is really easy. Simply follow the steps below:
Go to each post in “Edit” mode. In the right side of the page – click on “Set Featured Image“.
This will trigger the image uploader. Upload your image and as soon as that’s done, click the “Use as Featured Image” link:
Note that you should keep the file sizes to a smaller size. The resizing script has a maximum size limit. You can view the results in the single post page – here is a screenshot of Aparatus: