If you have applied a filter to your photo and pressed the update button you cannot then go back and revert to its original state. The way to get around this is when you have the photo in your dashboard, before you start to edit, mouse over the image and press the 5th icon which is duplicate – when you have two versions of the photo, move one of them to your media library by pressing the second icon. You can then go ahead and edit photo in dashboard, knowing you have a copy of the original in the media library.