iMessage capability is found only on apple devices that have an apple user iCloud ID associated with it.
if your friends have iCloud iDs and are using apple devices then sending by iMessage always results in the message being sent and received through iMessage.
and, as you know, you can send messages with or without so-called multimedia attachments.
i have a similar situation as you: my carrier plan does not provide any MMS capability whatsoever.
(my carrier is an MVNO that uses a major carrier's lines). therefore my carrier does not / can not provide me with what is called an "MMS mobile mail email address" to me.
when i want to send a message to a friend who is accustomed to always using MMS i can not.
i need to send him an email or an SMS.
likewise, he can not send an MMS to me, because i have no MMS mobile email address.
the above is the way it works with my carrier, and, in general, in japan.
ymmv
thank god MMS is almost dead, anyway.