Please can anyone help me with the manipulation of PNG image files?

I have a requirement to be able to create a template document in the RTF format, and then to switch in different company letterheads that are saved as PNG format files.

If you insert any graphic image into a Word document, then Save As... and select RTF as the output format, Word automatically converts the image to a PNG format. However if you look at this .RTF document in a text editor, the graphic data does not match the format of a .PNG file also opened in a text editor.

Any help or pointers gratefully received,
Regards
Simon