How to make a list of case studies with a thumbnail image?

Okay… so the word ‘cover’ could actually be anything – ‘moon_base’ – for example. So the word ‘cover’ is just the name/label that appears in the Panel above the field type. In that case I’m going to rename it ‘Key image’… just tried that and I get an error message saying “The field “key%20image” could not be found”.

Okay I remember, you can only use

alphanumeric characters and underscores in field names

Is there a way to get this label / name to be able to display more than one word in the Panel, without using underscores? For example ‘Key image’ or ‘Main text column’ etc to help clients to understand what is going on?

EDIT

Okay I can use a label to get more than one word to display in the Panel

subheading:
  label: Sub heading
  type: text

Don’t understand that. Does the first name relate to and link with the field names used in my content .txt files? But it doesn’t seem to match. In the Panel I have description_meta_tag but in my .txt file it is written Description-meta-tag