binarywriter to stream or binarywriter to bitmap

You cannot "convert" them. The whole notion makes no sense.

First: System.IO.BinaryWriter vs. System.IO.Stream. First class is a specialized wrapper (binary, write-only) over the second one. Please see:
http://msdn.microsoft.com/en-us/library/system.io.binarywriter.aspx[^].

Do I have to explain more? (Sigh…) Silence? I can see in your eyes that I better do. OK. You can construct a writer out of the instance of the Stream and/or later get this instance using System.IO.BinaryWriter.BaseStream:
http://msdn.microsoft.com/en-us/library/atxb4f07.aspx[^],
http://msdn.microsoft.com/en-us/library/d4w83hdt.aspx[^],
http://msdn.microsoft.com/en-us/library/system.io.binarywriter.basestream.aspx[^].

A Bitmap now? The System.Drawing.Bitmap is System.Drawing.Image, please see:
http://msdn.microsoft.com/en-us/library/system.drawing.bitmap.aspx[^],
http://msdn.microsoft.com/en-us/library/system.drawing.image.aspx[^].

You can use Image methods (see above) to work with streams; please locate FromStream and Save method in the MSDN article referenced above.

That's it. No "conversions".

—SA