Thumbs.db is a file created by Windows when thumbnail view is used. It is a hidden file not viewed by most users and not updated when files are moved from a folder which images have passed through or deleted. This gives a secondary chance that someone will leave behind at least partial evidence of an image in their Windows folders.
The thumbnails in Thumbs.db are stored in the OLE Compound File format. It's the same format that Microsoft Office uses.
There is a forensic open source application developed at sourceforge called vinetto at https://sourceforge.net/projects/vinetto that can extract them. It does require a python environment. Additionally, there are several other Java solutions based around the Jakarta project that is apart of Apache. Additional resources about thumbs.db can be found in a white paper at https://www.exterro.com.
MiTeC Windows File Analyzer 1 is a tool for forensic analysis of Thumbnail Databases, Prefetch files, shortcuts, IExplore Index.DAT files and Recycle Bin contents on a Windows system. It will print a report of analyzed files.
Windows Vista/7
See Vista thumbcache
Thumbs.db no longer exists in Vista/7 as individual files. This data has been moved to a centralized database located in \Users\%username%\AppData\Local\Microsoft\Windows\Explorer
Windows Vista will save thumbnails for files on mounted encrypted file systems (except EFS.
In Windows 7, thumbs.db files are not created under normal usage when a user browses a folder in explorer. However, they are created if the same folder is browsed through its network path such as via \\localhost\c\$. These files do not follow the xp thumbs.db format. The format is still an OLE container but without the Catalog stream and with a few other changes.
Windows 8/8.1
In Windows 8 and 8.1, both thumbcache and thumbs.db are present. Thumbs.db will only be created in folders which reside under a user profile folder (C:\Users\USERNAME\*). But accessing other folders using their network paths such as \\localhost\c\$ will create thumbs.db in other folders as well. The format of these thumbs.db files is the same as Windows 7 thumbs.db.