Thumbs.db is a file used to store thumbnail data of a folder in Windows. It makes loading of thumbnails significantly faster, however it has some privacy issues. Like if you delete an image, its thumbnail might still be there in thumbs.db file which would allow you to view it.
There’s an open source python script called Vinetto available to extract all the thumbnails in open.db to a folder. It works on Linux, Windows and Mac OS. You can download the script from here.
On Ubuntu, you can use apt-get to install vinetto:
sudo apt-get install vinetto
To extract thumbnails:
vinetto -o ./thumbnails ./thumbs.db
This will extract all the thumbnails inside thumbs.db to thumbnails folder.