By default, thumbnails are 96×96 pixels in size. You can change this value for the current user or all users. Current user: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer All users: HKEY_LOCAL_MACHINE\Software\Microsoft\Windows\CurrentVersion\Explorer Go to either key, select Explorer, then create a DWORD value called ThumbnailSize. Double-click it, select Decimal in the Base section, and enter a value between 32 and 256. Restart Windows Explorer.
Tried the above tips, really work! I put 169 as the decimal value, now my explorer shows 9 (3×3) thumbnails. Can see clearly the content of the pictures.