Many themes use “Featured Image” as a way for administrators to choose which image represents a post. Frequently the Featured Image will be the thumbnail that represents a Post on a Category table-of-contents page (this is how it works for WPFolio). But they can also be used in slideshows for featured Posts and many other things.
To choose a feature image, first select the appropriate post from the All Posts tab on the left toolbar (or start a new one, by selecting Add New).
Click the Add Media button.
If the image is not already in your Media Library, click “Upload Files” and then “Select Files” and choose the appropriate file on your computer. Be sure to choose “Set Featured Image” from the left hand menu. Once you have selected the image, click the “Set Featured Image” button at the lower right.
WordPress will create multiple sizes of an image, including a thumbnail.
Select the image for your Featured Image and then click “Set Featured Image” button at the bottom right.
To remove the featured image click “Remove featured image” link and then repeat the process to replace the featured image.