Windows thumbcache -
Overview
Windows Vista stores thumbnails in the following directory:
\Users\%username%\AppData\Local\Microsoft\Windows\Explorer 
This directory contains following files:
thumbcache_idx.db
thumbcache_32.db, thumbcache_96.db, thumbcache_256.db, and thumbcache_1024.db
thumbcache_sr.db
Thumbnails are stored in thumbcache_NN.db files in different formats. There are several tools that can work with Vista thumbcache database (see below) and individual images can be extracted using file carving. Unfortunately, there is no information in the thumbcache database that can easily link thumbnails with original files in all cases. One of the ways to link the thumbnails with original files is to use the contents of the Windows Search (windows.edb) database.
Thumbcache Format
In general, every thumbnail in cache is associated with two 64-bit variables. First variable (sometimes called Unique ID, Secret, File ID) associates data in file thumbcache_idx.db with thumbnail data in thumbcache_NN.db files; the purpose of this variable is unclear. Another variable is Thumbnail Cache ID (sometimes called Thumbnail filename (in FTK is used to link thumbnails with original files. Actually, Thumbnail Cache ID is represented as Unicode string of HEX encoding.
Thumbnail Creation Process
Windows Vista creates thumbnails for files on different media types, including:
Removable devices
Network drives
Encrypted containers (e.g. PGP Desktop, TrueCrypt, BestCrypt)
Windows Vista doesn't create thumbnails for files encrypted using EFS unless thumbcache directory is encrypted too; Windows Vista doesn't delete thumbnails for files after they were encrypted using EFS.
Some programs may generate thumbnails for some file types which are displayed in Windows Explorer, but not stored in the thumbcache (e.g. Ascon Kompas).
Linking thumbnails with original files
Using Windows Indexer
One way to link thumbnails with original files is to use Windows Indexer database, which stores association between indexed files and ThumbnailCacheIDs with some metadata. The windows.edb database file contents can be extracted using Windows Search Index Extractor
Using Windows PowerShell
Windows PowerShell provides easy way to access this database using SQL queries. Note that most forensic tools (like FTK display ThumbnailCacheID (where FTK calls it Thumbnail filename) in hexadecimal, but Windows PowerShell returns the result in decimal.
Using HEX editor
You can also search for ThumbnailCacheID value in Windows.edb file using your favorite HEX editor.
Vista Windows Photo Gallery
Windows Vista includes a built-in picture previewing tool called Windows Photo Gallery (the LIVE edition may also be installed by the user). Both of these programs create the files pictures.pd4 and pictures.pd5 respectively containing the ThumbnailCacheID and file path information of previewed pictures and videos. The contents of the pictures.pd4 and pictures.pd5 can be extracted using WPG Viewer
External Links
Windows Explorer Thumbnail Cache database (thumbcache.db) format, by the libwtcdb project
ThumbnailCacheId
System.ThumbnailCacheId
IThumbnailCache interface
Non-English
Использование централизованных баз данных эскизов для исследования графических файлов на зашифрованных разделах, ITDefence, 2009 (extended version)
Forensische Analyse des Microsoft Windows Thumbnail Cache, by Stefan Augustin, August 2023
Tools
FTK
Thumbs.db Viewer
Thumbcache-viewer
WinThumbs