you actually cant use MMS with jailbreaking
you used to be able to, but the application no longer works
and also, you had to have mms on your plan, which, being an iphone customer, you dont
jailbreaking has lots of positives, but also lots of drawbacks
it messed my phone up 3 times, once permanently freezing it so that it required a restore, and once possibly (though im not sure if this is why, i believe it is because of the strain on the hardware from jailbreaking or major firmware changes, as the same problem is thought to be caused by upgrading to 2.0) giving my phone an infamous dead strip