Add a featured image for a post or page
1. Click on the Add Featured Image link in the bottom right corner of the post editor

2. Upload a new image or select one from the Gallery

3. Click on the Use as featured image link at the bottom of the pop-up box

To use this image on the Home Slideshow you must check the box that says feature this on the home page

Update, or Publish

To learn more about creating a slideshow, see Creating a Slideshow