Unable to download photos from my iPhone onto iPhoto on my MacBook

Suddenly I am unable to download photos from my iPhone to iPhoto 9.6.1 on my MacBook Pro. When I plug my iPhone into my MacBook Pro, I click on Import Photos (as I always have done) and I get a message saying "Error downloading image." It says "iPhoto cannot import your photos because there was a problem downloading an image." What do I need to do?