You won't be charged for "this" specifically, but you'll be charged for everything else that happens in the background at the same time, sadly :( The effect will be minimal if you're on WiFi while doing this, but still enough to trigger data charges.
Sending pictures is included in your unlimited messaging plan. Data does have to be turned on for pictures to go through. You are not charged for that. But, if you have any app open, running in the background, they will try to update and pull in info. For example if you left the weather app running it will try to get the most info. So, turn all apps off to avoid the "leakage".