Adding to an old post, because I too have this problem.
I have imported two jpg files, and dropped them into my timeline. The thumbnails are black squares under the imported files tab, the clip preview, the sequence preview and the timeline. I can see the image in the timeline under the clip preview.
I have a reistered version of Videopad Vido Editore Professional v3.29, and have reinstalled this software. The problem remains.
Thanks
Ian