If you have a live site and a stage (or dev) site, you will probably want to add any new files (such as images) from the files directory on the live site to the staging site, or vice versa.
Run this rsync command, assuming you have navigated to the root Drupal directory (such as public_html). This also assumes that your stage site is located in the stage directory.
-r recursive - will include subdirectories -v verbose - will display the files on the screen as they move
rsync -rv sites/default/files/ stage/sites/default/files/