Issue in iMovie when replacing images.

When I drag an image from the iPhoto browser onto another image already in the timeline and select replace, there is a blank screen in the video preview where the newly replaced photo should be. This also occurs when deleting a photo and dragging another in it's place. I have tried exporting the movie to see if the picture shows up then, unfortunatly, the same issue still remains! How can I solve this? 😕