I dont have a Mac, so I cant say for sure, but you should be able to attach your picture with the attachments function, just below where you type after you create a new message. The attachment(s) will automatically show up to people who read the message. 

You might have to re-size your picture to keep it on the screen, or to make it small enough to get in under the size limit. How to do that on a Mac is beyond my experience.