how can i put a thubnail to the new template created

Hello.

I want to know how can i shows a thumbanil of the template a create to use with the composer in a new entry.

The entries template availables by default has a thmubnail that allow the user to know or have an idea about how the template design look, but i can´t find where can i put the thumbnail of new templates created...

Ill let you an image in order to be more clear.

Ill wait for your answer.

thanks!