Hi there,
you can save the image in a Custom Field, which WP has built in… but it has a terrible UI. See here.
A better alternative would be the ACF plugin as it has a “nicer” UI for adding the images. The free version will do.
Checking your site, its constructed using GP Elements and GB, so the Image block you have currently set to display your featured image, could be set to display an image using the Post Meta Content Source, and setting the Custom Field name.