The shortcode method, I believe, is for displaying the value inside the post (in the post content).
The more traditional way is to use WordPress codes - such as
the_meta - to call in your custom fields within your theme's templates, but outside the body of the post. Doing this, no one does anything special to "display" the fields when you write a post. Just fill in the custom fields (fill in the email box of Custom Fields Templates), and it will appear where-ever you have placed
Note that the_meta displays all Key-Value combinations.
Thumbnail image -
If you just want to display a particular one, then consult the specific codes in the links MichaelH gave, or try another plugin: Get Custom Fields plugin.