Missing image

When I try and create a video, before it begins it comes up with a message to the effect that there is a missing image (which it names) and asks me if I wish to continue. If I say "Yes" it all seems to work fine and I can see no blank images in the final video.

However, going through all the slides, I can find no reference to the missing image, nor see a "grey" blank missing image.

So my questions are...

1. Why is it doing this?
2. Is there a way to find out what slide is supposedly missing the image?
3. How can I fix this problem?

FYI - I have the latest version of SmartShow3D and have been using it for some time now and have never come across this before.