IOS – Delete iPhone MMS Messages Only

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.

Yes, videos and photos are stored in messages and they take up too much space. Try deleting any large discussion thread you do not need if you would like to free some space. Or, if you would like to weed out individual photos or videos, you can tap and hold on any message bubble for the context menu. It will appear with two options: "Copy" | "More". Tap "More" for a list of checkboxes to appear on the left side of the screen. This allows you to easily delete one or more (large) individual messages.

Up to iOS 6 messages just weren't in the usage tab, and consequently one couldn't easily see how much space it consumed.

Also note, that you are just deleting those messages off of that device. If they are iMessages, and you have other devices (a Mac or OSX device) with those accounts synced there, the messages will stay there.