param ([string]$path,[string]$ext)
function Refresh-Explorer
{
   $code = @'
   [System.Runtime.InteropServices.DllImport("Shell32.dll")]
   public static extern Int32 SHParseDisplayName([MarshalAs(UnmanagedType.LPWStr)] String pszName, IntPtr pbc, out IntPtr ppidl, UInt32 sfgaoIn, out UInt32 psfgaoOut);

   [System.Runtime.InteropServices.DllImport("Shell32.dll")]
   private static extern int SHChangeNotify(int eventId, int flags, IntPtr item1, IntPtr item2);

   [System.Runtime.InteropServices.DllImport("Shell32.dll")]
   private static extern int ILFree(IntPtr pidl);

   public static void Refresh(string path)  {
    uint iAttribute;
    IntPtr pidl;
    SHParseDisplayName(path, IntPtr.Zero, out pidl, 0, out iAttribute);
    SHChangeNotify(0x00002000, 0x1000, IntPtr.Zero, IntPtr.Zero);
    ILFree(pidl);
}
'@

    Add-Type -MemberDefinition $code -Namespace MyWinAPI -Name Explorer
    [MyWinAPI.Explorer]::Refresh($path)
}

cls
if([System.String]::IsNullOrEmpty($path))
{
   Write-Host "Path cannot be empty."
   Write-Host "Example:  .thumb_generate.ps1 -path ""C:"" -ext ""jpg"""
   return
}
if([System.String]::IsNullOrEmpty($path))
{
   Write-Host "Extension cannot be empty."
   Write-Host "Example:  .thumb_generate.ps1 -path ""C:"" -ext ""jpg"""
   return
}

$fileExtension = "*." + $ext
Write-Host -ForegroundColor Green "---Thumbnail generation for Windows 7/8---"
Write-Host -ForegroundColor Green "----PowerShell 3.0 & .Net 4.0 required----"
Write-Host "Path: "  $path
Write-Host "Extension: " $fileExtension
Write-Host
if (Test-Path $path)
{
   Write-Host "Path Exists, begin generation thumbnails"

   $images = [System.IO.Directory]::EnumerateFiles($path,$fileExtension,"AllDirectories")
   Foreach($image in $images)
   {
       try
       {
          $file = New-Object System.IO.FileInfo($image)
          Write-Host $file.FullName
          $fStream = $file.Open([System.IO.FileMode]::Open,[System.IO.FileAccess]::Read,[System.IO.FileShare]::None)

          $firstbyte = New-Object Byte[] 1
          $result = $fStream.Read($firstbyte,0,1)
       }
       catch
       {
          Write-Host -ForegroundColor Red "An error occured on file: " + $file.FullName
       }
       $fStream.Close();
   }
    Refresh-Explorer
}
else
{
  "Path Doesn't Exist, Exiting..."
}

.thumb_generate.ps1 -path "C:PathToImages"  -ext "jpg"

public static void refreshThumbnail(string path)
{
    try
    {
        uint iAttribute;
        IntPtr pidl;
        SHParseDisplayName(path, IntPtr.Zero, out pidl, 0, out iAttribute);
        SHChangeNotify(
            (uint)ShellChangeNotificationEvents.SHCNE_UPDATEITEM,
            (uint)ShellChangeNotificationFlags.SHCNF_FLUSH,
            pidl,
            IntPtr.Zero);
        ILFree(pidl);

    }
    catch { }
}

CoInitialize(NULL);

IThumbnailCache *cache;
HRESULT hr = CoCreateInstance(CLSID_LocalThumbnailCache, NULL, CLSCTX_INPROC, IID_PPV_ARGS(&cache));
if (SUCCEEDED(hr))
{
    <loop over files>
    {
        cache->GetThumbnail(<shell item for file>, <required size>, WTS_EXTRACT, NULL, NULL, NULL);
        cache->Release();
    }
}

mkdir thumbjunk
for f in *.jpg
do convert $f -thumbnail 100x80 thumbjunk/$f.gif

find * -prune -name '*.jpg'
-exec convert '{}' -thumbnail 100x80 thumbjunk/'{}'.gif

ffmpeg -ss 00:01:00 -i testavi.avi -vf 'select=not(mod(n,1000)),scale=320:240,tile=2x3' testavi.png