you can't do it from the mobile site unless you've uploaded it to tinyurl or similar, in which case you can do [ img]URL[ /img] (with no spaces).
If you go to the desktop version of the site ON your mobile device (link at the bottom of the page), you can click the little "add attachment" button and add an attachment, so upload directly to proboards. Then hit "insert" and "ok", and the pic should be in your post.