iPhoto problems

I'm somewhat slow catching on to many computer-related things, but have been using iPhoto for a few years now. Increasingly, when I click on a photo, it starts to be displayed, but immediately the screen goes black with an exclamation mark displayed in a triangle in the middle of the screen. Does anyone know what causes that and what the remedy might be? I'm unable to find any information about that in my Macs For Dummies book. Thank you.