Lol...I just tried it from my phone...
Click on the Go Advanced. Then scroll down a bit and click on manage attachments. It will open a new window where you can choose the file from you phone gallery (I have an android, so not sure about iphone). Then click upload to the right. Once that's done, click to close the window (button at bottom). That should take you back to the posting screen and you should see that your pic is now attached. You should be able to submit your post then and your pic will show.