Nik,
Go up to the top right of your screen on the tool bar and click on "Settings."
Now on the left side or your screen under "My Settings," look down to "Miscellaneous" and click on "Attachments."
You'll now have a screen that shows you all of your posted attachments. Just click on the ones you want to delete or use the delete page command and delete all on that page when you click at the bottom of the screen on: "Delete Selected."
That's it.