Glad you asked. Not everyone knows this.
Hit your text icon and pop in your best friend's name in to 'To:' bar.
Then touch the message bar. Next to it is the camera icon and the App Store icon. Hit the App Store icon and a load of icons will come up on the base bar of your screen. Go for the red one with the magnifying glass on it. Press and then press find images in the bar. Just press the image you want and it pops up into your text message. 'Yay' is the most searched for category. Followed by 'annoyed', and 'omg'. Sounds about right.