This pretty much is not a LISTSERV issue (though subscribers will most
likely need to support MIME attachments, have enough mailbox space to
support graphic files, have enough bandwidth via their ISP, as well as to
not treat the message as a virus/spam).

SET listname MIME

would probably be the only LISTSERV list subscriber option, which may
already be the default. Also,

LANGUAGE= HTML
DEFAULT-OPTIONS= MIME

Specifying NOHTML will prevent the inline graphic and instead it will be
a "clickable" attachment.

/Pete