This indicates a disconnect between the Thumbnail and the original
this can be caused by two things
1 - User action deleting items from inside the iPhoto library - can be directly - can be from the All images list (never delete from there) - can be from running cleanup software like MacKeeer, CleanMac2 or other dangerous, defective software that improperly deletes or changes things in the iPhoto library - if you have done any of these things you must restore your backup from before you did them and damaged your iPhoto library - and I recommend getting rid of any "clean up software" ASAP (although being essentially MalWear it is not easy)
2 - corruption of the iPhoto library - If it is not #1 then Back up your iPhoto library, Depress and hold the option (alt) and command keys and launch iPhoto - rebuild your database
LN