Start typing the name of the contact you want to send a message to, and i OS will offer to complete it for you. If the name is blue in the auto-suggest field, they're on i Message and you can send them a message without paying a penny.
If the name is grey - and then displays as red when you select it - they are not (they're probably on Android) and you'll find that you get a 'Not Delivered' error message, or won't be able to activate the send button at all. Search the App Store for texting apps, and you'll find plenty of free offerings.
Try sending a tweet instead, which while limited on characters does allow you to attach images.
The average Twitter user will have notifications set up so they're immediately alerted to direct messages or mentions.
Video chatting's all the rage right now, and it's only going to get more popular as front-facing phone cameras become more common.