Windows keeps a copy of all of your picture, video, and document thumbnails in a cache database so they can be displayed quickly when you open a folder. Sometimes due to thumbnail cache corruption, thumbnail previews may not work for some files, for a particular icon size or for all icons sizes.
Thumbnail preview problems can be resolved by clearing the thumbnail database and let the system rebuild it as required. To reset the thumbnail cache in Windows Vista, 7, 8 and Windows 10, use these steps:
Clearing the Thumbnail Cache using Disk Cleanup
1. Click Start, type Disk Cleanup, click Disk Cleanup from Search results. Alternately, run cleanmgr.exe directly.
2. Click My files only (This step is for only for Windows Vista)
3. Select the system drive letter from the drop-down.
4. Uncheck all the entries, except Thumbnails
5. Click OK, and click Delete Files to confirm.
6. Although not always necessary, but sometimes you have have to logoff and login back after running the cleanup, for the change to take effect.
The thumbnail database is stored globally in Windows Vista and higher, unlike Windows XP, where the thumbnail database was stored on a per-folder basis. In Windows Vista and higher, the default thumbnail cache location is mentioned below:
If you plan to clear them manually, close all running programs, open a Command Prompt window, exit the shell, and do the deletion via Command Prompt.