I don't know if this will help, but I tried Tweakker on a giffgaff SIM in a fully jail broken Orange ZTE Blade I and it worked.
It creates a new APN named Tweakker which seems to have all the same settings as the ones recommended by giffgaff except the APN type is set to
Rather than giffgaff's setting of just default.
Maybe you could manually edit your APN type setting to match?
Of course it could have changed some other setting I didn't notice and it may not work for Orange.
It's worth noting that, up till a couple of weeks ago I was with Orange, and before I jail broke it in January, the basic San Francisco would send and receive MMS on Orange.
I didn't try to send any MMS in the next 3-4 months, only once I'd switched to giffgaff, so I couldn't say if it was the jail break or the giffgaff SIM that caused the problem.
I installed this ROM in January and everything was fine until I switched from Orange to Giffgaff because of Orange's recent price hikes.
I can't say if MMS messaging was working with Orange after the jail break - I never sent a message with them, but it certainly isn't now.
I've opened a topic on the Giffgaff forum, but no one has come up with a solution.
As you can see I've tried all the usual suspects.
One question, is there anywhere in the menu structure I can check / change the maximum MMS message size?
4 days later.
I found the following topic on these forums
There is a suggestion that installing the Tweakker app will fix MMS problems on some UK networks (Orange, giffgaff etc)
It worked for me, I can now send and receive MMS messages.
However the original poster of the topic seems to have found it didn't work for him on Orange.
Thanks for this, it worked a treat.
BTW there are two other places you can find the IME. It's on the box label, if you've still got it and on the label in the phone under the battery which you have to remove in any case to swap the SIM.
Although you are entering your IME on an unknown web site, they do not ask you for any other details, so it can't be tied to you or your phone unless you elect to go to other parts of their site (which I didn't) and possibly tell them.
Theoretically it could be tied to the URL supplied by your ISP, but that's as far as it goes.