There are some causes for this issue, but the most usual one is related to the file path or file integrity. Check the files you want to add because it's possible they're corrupted. Displaying won't be possible even if you burn them to a DVD. However, there is another way to test this. Leave the product as it is and then export the slideshow to some other format. A Youtube video, general video, or even an EXE file should do the trick. Export to one of these formats and observe the results. If the images don't appear correctly, replace them and check the source of the files.
