Sangoma support just replied to a ticket I had opened about this; they said MMS isn’t supported on SipStation trunks.
Here is a FreePBX Wiki article as proof that its supposed to work: https://wiki.freepbx.org/display/FPG/UCP+SMS
Here is another forum post about a similar issue: SIPStation Send/Recieve Images not working
Edit: Pointed them towards the above links and it turns out it is supported. Will try to keep community updated on progress and fix.