Automator, as mentioned, can make short work of these kinds of tasks. Have you used Automator before? It's actually quite easy.
1. Tell Automator where to get the stuff to work on (files, songs, etc.)
2. Tell Automator which actions to perform on the above items, by dragging things from the left pane into your workflow
3. Run it.
Have a look through the available actions - I believe Preview can crop images, and there's a built-in Automator action for doing this.
I found a third-party Automator Action that might help you. Have a look here.