iMessage lets you easily send photos without paying your carrier for a special messaging plan (data rates will still apply if you aren't on Wi-Fi). iMessage is built right into the same Messages app and Share Sheet that sends SMS and MMS to your non-iPhone using friends, so you don't have to do anything extra to use it. Just pick your photos and send them!
That's all there is to it, your photo is on its way. Alternately, you can tap on more than one photo in order to send more than one at a time.
If you're having issues getting iMessage to activate, get more help at the iMore forums, and for more tips, check out: