First of all, stay out of the finder. Mucking about in there for your photos is a sure way to corrupt your library and make your photos inaccessible (if you use iPhoto,that is). iPhoto stores photos in its own way so that it can access them for you.
Now, to find your original, go into iPhoto, find the photo you want the original of and through the menubar (I am not at the mac right now, so can't say specifically which menu -- probably "Photo") and find "Revert to Original". Then you should have your original photo back. If you want to retain the modified photo as well, make a duplicate of it, then revert to original.