Change default storage location of Screenshots
You can achieve this using Foldersync app.

Add folderpair and select account: SD CARD.
Then you can select source by finding your screenshot dir and select target dir on your SD.

It also has full range of advanced options if you don't like how default options work so everything is covered.

Bonus tip: this app can also do backups of your SD (or internal) card to remote storages such as Dropbox, Google Drive, Owncloud, etc.