Changing Photo thumbnails
Since it's only one photo you might find it easiest to Export it from iPhoto (as a .png perhaps) as "Current", then open it in Preview and rotate it four times, then Save it (if it's .png then you won't lose quality). Preview should have made a new thumbnail for you— now you can re-import into iPhoto and check to make sure everything is ok. You will have to redo location, Faces, and tagging info. But if everything is satisfactory then you can delete the messed up version you had before.

If you have lots of images with wrong thumbnails then you should launch iPhoto while holding CMD Option and rebuild the database.