How do you change the size of the images sent from an iPhone using MMS? I have an iPhone 4, my wife has a jailbroken iPhone 3G on Simple Mobile (T-Mobile's network).
She's able to send and receive MMS messages, but images that are taken on my iPhone are flagged as too large and she receives a message that she has received an image from me but it has been blocked due to size.

Older phones and other limited plans have this implementation, so this question should be useful to others.

Is there a way to change the size of images when sending them via MMS like you can when emailing a picture on the iPhone?

If not, how can I work around this?
One option is to add your wife's MMS e-mail address to her contact card and use that e-mail address to e-mail the picture to her phone.  When you send an image via e-mail, the iPhone will ask you what size you want to send (how much compression you want to do to the image). Choosing SMALL may allow it to go through. 
Here are a list of some SMS/MMS gateways that allow you to send a SMS/MMS via e-mail address:  List of SMS Gateways

You could also try using the free Adobe Photoshop Express app in the iTunes App Store to crop the image and save as a new image then try sending the smaller sized (both in dimensions and in bytes) image. A bit of a hassle, but the Messaging app does not have any built-in selectors for specifying the size.