Can’t figure out custom fields

I’m having trouble figuring out how to use custom fields. My theme has a “featured post,” but to display the image I need to use the custom field. I know the name of it is “Leadimage”, but I don’t know what to put in the “value” field. I don’t know how to link to the image that i want to use. Any help?

I’ve tried just uploading an image and putting the file URL in the value, but it isn’t working.

My site: https://www.lifetwitch.com/