If Recuva is supposed to securely overwrite checked/highlighted files, why do the previews still show up after reboot & rescan ?
Basically that’s because the image previews/thumbnails are not the image files that you have deleted.
The previews.thumbnails are stored in a Windows cache, Windows will remove the ones for deleted files after a while.
Running Windows Disk Cleanup or using CCleaner to clear the thumbnail cache should remove them straight away.