Featured photos
How to feature photos in my Gallery
Photos is supposed to do that for you automatically. It will come up occasionally with a photo you have not looked at in long time and add it to the featured photos. Give Photos some time to analyse the photos in your library.
You cannot add featured photos on your own.
If you want to show a photo you like in a widget on your Home Screen, you may want to try Widgetsmith. It is a bit involved, but you can create a widget. that will show the photos from an album you specify.