source: https://gallery.technet.microsoft.com/scriptcenter/Convert-Imageps1-028994f2

Convert-Image.ps1

Function Convert-Image
{
	<#
	.NOTES
		Author: Randy Turner
		Version: 1.0 - 06/16/2019
		
	.SYNOPSIS
	    Resize and optionally convert an image file.

	.DESCRIPTION
		The Convert-Image cmdlet will set a new size, resolution, and\or convert an image file.
		----------------------------------------------------------------------------------------
		Security Note: This is an unsigned script, Powershell security may require you run the
		Unblock-File cmdlet with the Fully qualified filename before you can run this script,
		assuming PowerShell security is set to RemoteSigned.
		----------------------------------------------------------------------------------------

	.PARAMETER FullName
		Specifies an input image file. 

	.PARAMETER Destination
		Specifies a destination for resized file(s). Default is current location (Get-Location).
	
	.PARAMETER SaveAs
		Specifies an output image type allowing type conversion.

	.PARAMETER WidthPx
		Specifies a width of image in pixels. 

	.PARAMETER HeightPx
		Specifies a height of image in pixels.

	.PARAMETER DPIWidth
		Specifies a vertical resolution. 

	.PARAMETER DPIHeight
		Specifies a horizontal resolution.

	.PARAMETER Overwrite
		Specifies a destination exist then overwrite it without prompt. 

	.PARAMETER FixedSize
		Set to a fixed size and do not try to scale the aspect ratio.

	.PARAMETER DetectLandscape
		Swap Height and Width when Landscape source image is detected. 

	.PARAMETER RemoveSource
		Remove source file after conversion. 

	.EXAMPLE
		PS C:\> Get-ChildItem 'H:\Test\TestIn\*.jpg' | Sort-Object -Property FullName | Convert-Image -Destination "H:\Test\TestOut" -WidthPx 800 -HeightPx 800 -DetectLandscape -SaveAs Png -Verbose
		VERBOSE: Image 'H:\Test\TestIn\1.jpg' was resized from: 600x598 to 800x797 and saved as: 'H:\Test\TestOut\1.png'
		VERBOSE: Image 'H:\Test\TestIn\2.jpg' was resized from: 600x598 to 800x797 and saved as: 'H:\Test\TestOut\2.png'
		VERBOSE: Image 'H:\Test\TestIn\3.jpg' was resized from: 600x598 to 800x797 and saved as: 'H:\Test\TestOut\3.png'
	#>
	[CmdletBinding(
		SupportsShouldProcess=$True,
		ConfirmImpact="Low"
    )]
	Param
	(
		[Parameter(Mandatory=$True,
			ValueFromPipeline=$True,
			ValueFromPipelineByPropertyName=$True)]
		[Alias("Image")]	
		[String[]]$FullName,
		[Parameter(Mandatory = $False)][Alias('Save')]
			[ValidateNotNullOrEmpty()]
			[ValidateSet('Bmp','Gif','Jpeg','Png','Tiff')]
		[String]$SaveAs='',
		[String]$Destination = $(Get-Location),
		[Int]$WidthPx,
		[Int]$HeightPx,
		[Int]$DPIWidth,
		[Int]$DPIHeight,
		[Switch]$Overwrite,
		[Switch]$DetectLandscape,
		[Switch]$FixedSize,
		[Switch]$RemoveSource,
		[Switch]$DetailedReport
	)

	Begin
	{
		Add-Type -A System.Windows.Forms
		$ImgTypes = @{Bmp = '.bmp';Gif = '.gif';Jpeg = '.jpg';Png = '.png';Tiff = '.tif'}
		$ImgGUIDs = @{'B96B3CAB-0728-11D3-9D7B-0000F81EF32E' = 'Bmp';
					  'B96B3CB0-0728-11D3-9D7B-0000F81EF32E' = 'Gif';
					  'B96B3CAE-0728-11D3-9D7B-0000F81EF32E' = 'Jpeg';
					  'B96B3CAF-0728-11D3-9D7B-0000F81EF32E' = 'Png';
					  'B96B3CB1-0728-11D3-9D7B-0000F81EF32E' = 'Tiff'}
	}

	Process
	{

		Foreach($ImageFile in $FullName)
		{
			If(Test-Path -Path $ImageFile)
			{
				$OldImage = New-Object -TypeName System.Drawing.Bitmap -ArgumentList $ImageFile
				$OldWidth = $OldImage.Width
				$OldHeight = $OldImage.Height
				$H = $HeightPx
				$W = $WidthPx
				If($OldImage.Width -gt $OldImage.Height -and $DetectLandscape.IsPresent)
					{
					#Landscape Image Detected
					$H = $WidthPx
					$W = $HeightPx
					}

				If($WidthPx -eq $Null){$WidthPx = $OldWidth}
				If($HeightPx -eq $Null){$HeightPx = $OldHeight}

				If($FixedSize.IsPresent)
				{
					$NewWidth = $WidthPx
					$NewHeight = $HeightPx
				}
				else
				{
					If($OldWidth -lt $OldHeight)
					{
						$NewWidth = $W
						[int]$NewHeight = [Math]::Round(($NewWidth*$OldHeight)/$OldWidth)
						
						If($NewHeight -gt $HeightPx)
						{
							$NewHeight = $H
							[int]$NewWidth = [Math]::Round(($NewHeight*$OldWidth)/$OldHeight)
						}
					}
					Else
					{
						$NewHeight = $H
						[int]$NewWidth = [Math]::Round(($NewHeight*$OldWidth)/$OldHeight)
						
						If($NewWidth -gt $WidthPx)
						{
							$NewWidth = $W
							[int]$NewHeight = [Math]::Round(($NewWidth*$OldHeight)/$OldWidth)
						}
					}
				}

				$ImageProperty = Get-ItemProperty -Path $ImageFile
				$SaveLocation = Join-Path -Path $Destination -ChildPath ($ImageProperty.Name)

				$NewImage = New-Object -TypeName System.Drawing.Bitmap -ArgumentList $NewWidth,$NewHeight

				$Graphics = [System.Drawing.Graphics]::FromImage($NewImage)
				$Graphics.InterpolationMode = [System.Drawing.Drawing2D.InterpolationMode]::HighQualityBicubic
				$Graphics.DrawImage($OldImage, 0, 0, $NewWidth, $NewHeight) 

				$ImageFormat = $OldImage.RawFormat
				$ImgFormatStr = $ImageFormat.Guid.ToString()
				$ImgFormatStr = $ImgFormatStr.ToUpper()
				$ImgType = $ImgGUIDs.Item($ImgFormatStr)
				If($SaveAs.Length -ne 0 -and $SaveAs -ne $ImgType)
					{
					# Image type conversion requested!
					$SaveLocation = -join($SaveLocation.Substring(0,$SaveLocation.LastIndexOf('.')),$ImgTypes.Item($SaveAs))
					$ImageFormat = [System.Drawing.Imaging.ImageFormat]::$SaveAs 
					}
				$OldImage.Dispose()
				If($DPIWidth -and $DPIHeight)
				{
					$NewImage.SetResolution($DPIWidth,$DPIHeight)
				}

				If(!$Overwrite.IsPresent)
				{
					If(Test-Path -Path $SaveLocation)
					{
						$Title = "A file already exists: $SaveLocation"
						$ChoiceOverwrite = New-Object -TypeName System.Management.Automation.Host.ChoiceDescription -ArgumentList "&Overwrite"
						$ChoiceCancel = New-Object -TypeName System.Management.Automation.Host.ChoiceDescription -ArgumentList "&Cancel"
						$Options = [System.Management.Automation.Host.ChoiceDescription[]]($ChoiceCancel, $ChoiceOverwrite)	

						If(($host.ui.PromptForChoice($Title, $null, $Options, 1)) -eq 0)
						{
							Write-Verbose -Message "Image '$ImageFile' exists in destination location - skipped"
							Continue
						}
					}
				}

				$NewImage.Save($SaveLocation,$ImageFormat)
				$NewImage.Dispose()
                
				If($DetailedReport.IsPresent)
					{
					Write-Verbose -Message "Image '$ImageFile' was resized from: $($OldWidth)x$($OldHeight) to $($NewWidth)x$($NewHeight) and saved as: '$SaveLocation'"
					}
				Else
					{
					$FN = Split-Path -Path $ImageFile -Leaf
					Write-Verbose -Message "Image '$FN' was resized from: $($OldWidth)x$($OldHeight) to $($NewWidth)x$($NewHeight)'"
					}

				If($RemoveSource)
				{
					Remove-Item -Path $ImageFile -Force
					Write-Verbose -Message "Input Image: '$ImageFile', was removed"
				}
			}
		}

	}

	End{}
}