A Post Thumbnail (Featured Image) is an image that is chosen as the representative image for Posts, Pages or Custom Post Types.
When you are writing a post you can set a thumbnail by clicking on “Set featured image” (located in the bottom right corner).
You can now select to either upload a new image or select one from the “Media Library”. Once you have selected an image, click on “Set featured image”.
Regenerating Featured Images
Each theme comes with its own thumbnail sizes. This is an issue because all your selected thumbnails might show up in different sizes.
After you activate one of our themes we recommend that you use this simple plugin, Regenerate Thumbnails. You should install it and use it.
Make sure you have a backup before doing anything.