A Powershell Script that converts all png files recusivly in given folder (and subfolders) when the resulting jpeg is smaller than the png and the png does not contain transparency. Requires Image Magick to be available in PATH.

convert_png.ps1

$sourceFolder = "C:\path\to\pngs"
$sumSavedKb = 0

# Get all PNG files recursively
Get-ChildItem -Path $sourceFolder -Recurse -Filter *.png | ForEach-Object {
    $file = $_.FullName
    $jpgFile = [System.IO.Path]::ChangeExtension($file, "jpg")

    # Get the old file size (in kB)
    $oldfilesize = [Math]::Round((Get-Item -LiteralPath $file).Length / 1KB, 2)

    # Check if the PNG file has transparency using ImageMagick
    $hasTransparency = magick identify -format "%[channels]" "$file" | Select-String -Pattern "a"

    if ($hasTransparency)
    {
        Write-Host "Skipping $file because it contains transparency."
    }
    else
    {

        # Run the ImageMagick convert command with quality and compression
        magick -quality 70 -compress JPEG "$file" "$jpgFile"

        # If the conversion was successful
        if (Test-Path -LiteralPath $jpgFile)
        {
            # Get the new file size (in kB)
            $newfilesize = [Math]::Round((Get-Item -LiteralPath $jpgFile).Length / 1KB, 2)

            # Check if the new JPG is larger than the original PNG
            if ($newfilesize -lt $oldfilesize)
            {
                # Calculate the size saved
                $savedSpace = $oldfilesize - $newfilesize
                $sumSavedKb += $savedSpace

                # Print log statement with file sizes and saved space
                Write-Host "Convert $file to $jpgFile ($oldfilesize kB > $newfilesize kB), saved $savedSpace kB"

                # Remove the original PNG file
                Remove-Item -LiteralPath $file
            }
            else
            {
                # If the new JPG is larger, delete it and keep the original PNG
                Remove-Item -LiteralPath $jpgFile
                Write-Host "Skipped converting $file, JPEG is larger."
            }
        }
        else
        {
            Write-Host "Failed to convert $file"
        }
    }
}

# Print total space saved at the end
Write-Host "Compression finished, saved $([Math]::Round($sumSavedKb, 2) ) kB"