Matthew Ephraim released an open source wrapper for Ghostscript that sounds like it does what you want and is in C#.
Link to Source Code: https://github.com/mephraim/ghostscriptsharp
Link to Blog Posting: http://www.mattephraim.com/blog/2009/01/06/a-simple-c-wrapper-for-ghostscript/
You can make a simple call to the GeneratePageThumb method to generate a thumbnail (or use GeneratePageThumbs with a start and end page number to generate thumbnails for multiple seperate pages, with each page being a seperate output file), default file format is jpeg but you can change it, and many other options, by using the alternate GenerateOutput method call and specify options such as file format, page size, etc...
