Is it possible to send MMS from Journey Builder. If yes, how can it be done since Journey Builder only allows one to create SMSs.
Unfortunately it is not possible to send MMS messages through Journey Builder without creating a custom activity for it. There is no (and probably will not be any) OOB functionality for it.
The reason why there is barely any support for MMS-related options is because Marketing Cloud only supports sending MMS messages in the US.
There is an open Idea that you can vote on though.