Further to comments by Terence Devlin...
If you can select from the Menu >Photos>Revert To Original, iPhoto should replace the edited photo with the original image.
If you want to find an original image file using time machine, and iPhoto no longer recognises this image exists, you should be able to select a photo from the same batch, select from the Menu File>Reveal In Finder>Original File, and once the Finder window is open, click Time Machine in the Dock.
With Time Machine running, I expect that you should be able to work your way back through this normally hidden folder until you find the "extra" or "missing" image file in this iPhoto batch folder.