Yes, you can post a picture from your iPad into an already started thread if you have icons above the Quick Reply box. If you have the icons, the little tree icon is 3 icons to the right of the emoticon (smiley face) . Write your message, click on the little tree and follow the directions to retrieve it from your photo gallery. After you click upload photo, you need to click -Post Quick Reply for it to appear on QB.
If you do not have the icons, you need to go into Settings at the top of the QB page and make sure you have WYSIWYG clicked on I think.