Images in media library are missing

Hi All,
I used Snapshot to rip a sub site out of a network and re-install it in a new stand lone site.It took a few tries but did finally work well.
While my client was testing and confirming all is well, she noticed that the media previews in her media library were missing. the files show as a file, but no image and the images show in the front end.
I' assuming some of the thumbnail images are missing.
What is the best way to copy and reload them?

Thanks for all you do.