iMessage sends text messages, pictures, and movies over cellular data rather than through the traditional SMS and MMS protocols, which are used on bundled Android Phone text apps and on third-party texting apps. Unlike third party iPhone apps and many other bundled services, there is no option to disable iMessage sending over cellular data while keeping it enabled for wi-fi connections, and instead you’d have to turn the feature off completely and fall back to SMS. That certainly works to avoid additional cellular data consumption, but falling back to SMS can have it’s own problems if you don’t have a generous texting plan associated with the iPhone, making it a delicate balance.
Perhaps the better solution for those with capped data plans who get hit with overage charges due to heavy multimedia based iMessaging is to monitor the usage from time to time, and be sure to connect to a wi-fi network as often as possible to unload the data from cellular onto the wireless network. You can always cut back on sending photos and videos too, or selectively send them as an SMS instead if you have a texting plan to fall back on – just keep in mind the SMS/MMS protocol is fairly ruthless with compression and the quality of any image or movie sent over traditional MMS is going to be abysmal compared to iMessage.