If you use Windows File explorer and navigate to any folder with some wallpapers in it, does the wallpapers thumb nail show up properly?
If, yes, Please try purge and reinstall steps as mention here : [Link]. Just make sure to redownload the latest version from your account page. Reboot after the purge. Reinstall and reboot again after the installation. Retest and report back here.
Thank you,
Basj,
Stardock Community Assistant.