Why do some of my pic's change from the icon to a triangle with exclamation mark in it

I have a number of pictures in iPhoto that look fine in the evnets library as well as in the albums yet when I click on them to view them all I get is a dark screen with a triangle surrounding an exclamation mark. Any suggestions the cause and the remedy. They used to work fine and randomly have started doing this yet there is no connection between photos either time wise nor subject and they are in various albums, no consistency.