Upload the file to any of the millions of image hosting sites (I like imgur.com). Copy the URL of the direct .jpg or .png file.
Then on the forum in the reply area, click the Image icon. Paste the URL of the image.
If you look again at the screenshot of the app (and not his notification bar), you'll see he tested on a data connection.
Anyone in East Bay on T-mobile? I'm curious how coverage is in the Berkeley area