Are you using the latest (1.12.1) version? I can break down the step for you:
- Right click and type “export”, this will create export node. When you create export node a dialog will appear. This dialog allow you to select where to save. You can then click on “execute node” to save image.
or
- Right click on the preview and select “save current preview as…”. This will create the same dialog, where you can choose where to save image to.