For sites on your local, you can use `wp media regenerate --only-missing --yes`
For Pantheon sites, use Terminus to get CLI access to your site (even live!)
Format: `terminus remote:wp <site>.<env> -- <wp-cli command>`
Example: `terminus remote:wp american-rivers.live -- media regenerate --only-missing --yes`
[More details on the wp media regenerate command](https://developer.wordpress.org/cli/commands/media/regenerate/)