Use the upload icon second from the right in the compose window. It will get you a "browse" pop up and you can find your photo to post. It won't work if your photo file is too big, so downsize it first (save under another name). Cheap & dirty way to do this if you don't know how is email the photo to yourself, choosing "medium" when the phone prompts you for file size, then save the file when you get the email and upload it. Good luck!
You can check that it worked as you will see your photo in the split screen on your right (I am using a computer, so I'm not sure you get the same view on a phone).