When attaching an image to a MIME multipart mail (either multipart/related or multipart/mixed), you can give the image a Content-Id header with an arbitrary cid value.

In the HTML body of your mail, you can then reference those cids again to display images.

>  arbitrary cid value.

AFAIK should the content id be globally unique, some mail clients use it for caching.

It's a good idea to prepend a unique mail id

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery}