Setting a featured image for your posts is pretty simple in WordPress. What if you would like to add a secondary image though? Some themes may have that functionality already built-in. Thanks to Multiple Post Thumbnails, you can add such functionality to your existing theme.
Multiple Post Thumbnails is pretty simple to use. Once you have installed it, you will be able to select a main and secondary featured image for your posts. The plugin comes ready to go, but if you know coding, you can always get a bit creative with it. This is a free script.