Hi,
The drag & drop editor should automaticly create your thumbs. Please check via ftp if you have the following folders on your server:
- public_html/media/com_acym/templates
- public_html/media/com_acym/images
If they are not present, create them and test again.
Please note that Acymailing does not generate thumbs for html templates and that this includes imported templates. They are generated for the drag & drop editor only.