How can I create images from an existing PDF file?

Not at all with PDFsharp. PDFsharp can only create and modify PDF files. Rendering PDF is a completely different thing. It is not planned to add a PDF renderer to the PDFsharp library.

But a well-tried and reliable tool to do that is Ghostscript.

PDFsharp 0.9 comes with a new Assembly that contains a thin wrapper class around the Ghostscript DLL. This code shows how to use Ghostscript from a .NET application.