If you want to use an external .Net lib in powershell, here is a simple example...
$lib="C:\yael\YaelExample\bin\Debug\zrlabs.Yael.BasicFilters.dll"
[Reflection.Assembly]::LoadFrom($lib);
[System.Reflection.Assembly]::Load("System.Drawing, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a");
$img = [System.Drawing.Bitmap]::FromFile("c:/tempcat.jpg");
$textWaterMark = new-object zrlabs.Yael.BasicFilters.TextWatermarkFilter;
$transformedImage = $textWaterMark.ExecuteFilter($img);
$transformedImage.Save("c:/temp/cat_watermark.png");
Mostly a "note to self"...