My bmz_cache is getting big, especially for backups.

I went to the IH2 admin, and cleared cache "Image cache cleared" statement appeared, checked the bmz_cache folder, a file called .keep was deleted but the folders and images remain...Do I FTP delete them?

and if so, is this how to do it in future, delete the folders first, then clear cache in admin? not sure what clear cache in admin does then...