You probably want multipart/related (I can't remember which RFC specifies this), or, if the images are very small, "data" URIs (ditto for RFC).

I'd recommend using MIME parts for the images you want to appear with your text/plain alternative (you're not sending HTML-only, are you?) and "data" URIs for decorative elements that are not themselves part of the content.

In HTML, you're limited to about 700 bytes in a base64 data URI, because of LITLEN 1024 in the SGML Declaration for HTML.