I am not sure you can do this -- if you create HTML, then the PNG files are separate image files and they are NOT embedded.
If you create RTF or PDF, those generally can't be the "body" of the mail. So, although images are embedded in RTF or PDF files, the RTF files or PDF files would be attachments to the mail, not the main body of the mail.
You might want to work with Tech Support on this question. A lot will depend on the formats that your mail administrator allows you to send. For example, if you are bound to only use Plain Text for the mail, then you can't embed the PNG file in Plain Text and you can't embed the PNG file in HTML. So you have to find out your mail system limitations first.