Create Media item from file on server drive

New-MediaItem.ps1

function New-MediaItem{
    [CmdletBinding()]
    param(
        [Parameter(Position=0, Mandatory=$true, ValueFromPipeline=$true)]
        [ValidateNotNullOrEmpty()]
        [string]$filePath,

        [Parameter(Position=1, Mandatory=$true)]
        [ValidateNotNullOrEmpty()]
        [string]$mediaPath)

$mco = New-Object Sitecore.Resources.Media.MediaCreatorOptions
$mco.Database = [Sitecore.Configuration.Factory]::GetDatabase("master");
$mco.Language = [Sitecore.Globalization.Language]::Parse("en");
$mco.Versioned = [Sitecore.Configuration.Settings+Media]::UploadAsVersionableByDefault;
$mco.Destination = "$($mediaPath)/$([System.IO.Path]::GetFileNameWithoutExtension($filePath))";

$mc = New-Object Sitecore.Resources.Media.MediaCreator
$mc.CreateFromFile($filepath, $mco);
}

# Usage example
New-MediaItem "C:\Users\Adam\Desktop\Accelerators.jpg" "$([Sitecore.Constants]::MediaLibraryPath)/Images"

Update-MediaItem.ps1

function Update-MediaItem{
    [CmdletBinding()]
    param(
        [Parameter(Position=0, Mandatory=$true, ValueFromPipeline=$true)]
        [ValidateNotNullOrEmpty()]
        [string]$filePath,

        [Parameter(Position=1, Mandatory=$true)]
        [ValidateNotNullOrEmpty()]
        [string]$mediaPath)

[Sitecore.Data.Items.MediaItem]$item = gi $mediaPath
[Sitecore.Resources.Media.Media] $media = [Sitecore.Resources.Media.MediaManager]::GetMedia($item);
$extension = [System.IO.Path]::GetExtension($filePath);
$stream = New-Object -TypeName System.IO.FileStream -ArgumentList $filePath, "Open", "Read"
$media.SetStream($stream, $extension);
$stream.Close();
}

# Usage example - overwrite the image created with previous cmdlet with new image
Update-MediaItem "$SitecoreDataFolder\Tchotchkeys.jpg" "$([Sitecore.Constants]::MediaLibraryPath)/Images/Accelerators"