Google finds me several packages or program that run in Windows and can send MMS messages. But they all need a GSM phone (connected with Bluetooth or USB-cable) to actually send the message.
So I'd use a cheap phone with a prepaid SIM, that you only use for this purpose. Then it's easy to delete all incoming spam and telemarketers will find nobody ever answers, because you only switch it on to send an MMS and then turn it off again.
How can I send MMS messages from my computer?
I'd like to use websites that pop up in a Google search... but how do I know if I can trust them not to spam me or my recipient with junk texts, or sell my numbers to telemarketers?