Iphoto Images Display Half Black After 'rotate'


Many 'portrait' format images in my Photo Library which have previously been rotated from 'landscape' in iPhoto (and at first displayed correctly) now appear half black - ie the image displays perfectly but only filling one half of the image area (top half if now displaying as 'portrait' or left half if now displaying as 'landscape'), with the other half black.
Does this mean the file has been permanently corrupted, or might it be correctable ?
How might this have happened ?
How can I prevent it happening again ?

I'm running iPhoto 2 on a G4.

Thanks, Nigel