Something like this should work:

 public class WordGenerator : IDocumentGenerator
 {
         FileStream IDocumentGenerator.GenerateDocument()
         {
                object missing = System.Reflection.Missing.Value;
                Document adoc = new Document();
                adoc.InlineShapes.AddPicture(@"http://localhost:2014/Resources/MG.PNG", ref missing, ref missing, ref missing);

                // save the doc file
                object fileName = Path.GetTempFileName();
                adoc.SaveAs(ref fileName, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing);

                // quit word and release COM objects
                object saveChanges = WdSaveOptions.wdDoNotSaveChanges;
                adoc.Application.Quit(ref saveChanges, ref missing, ref missing);
                Marshal.ReleaseComObject(adoc);

                // return the stream
                return new FileStream((string)fileName, FileMode.Open);

         }
 }

and you'll have to delete the temp file sometime in the future.