Because Shotwell is a non-destructive photo editor, you can undo any alterations you make to a photograph.
If you want to see what a photo looked like before your modifications, press the Shift key. The original photo will be displayed as long as you hold the key down.
To completely undo all the changes you've made to a photo, click Photos ▸ Revert to Original. The only exception is that time and date adjustments will not be reverted.