If you want to post a picture on the ipad, I'd recommend getting the app 'Pickle'. It allows you to copy the URL of a picture on the ipad. Also, if it's a signature you want, you will probably need to click 'attach files + more reply options' and post your signature in some random thread, copy the URL, then edit the post to get your sig out of it. Next, you go into settings, signature, and then type in-
[ img = URL ] without the spaces, and the URL of your sig replacing 'URL'. Then you're done! However, if you want the sig centered, you type this code-
[ center ] [ img = URL ] [ / center ] without spaces. Hope this helped! :3